Device Operating
Temperature Range Package
HIGH PERFORMANCE
CURRENT MODE
CONTROLLERS
ORDERING INFORMATION
UC3844D
UC3845D TA = 0° to +70°C
TA = – 25° to +85°C
SO–14
SO–14
PIN CONNECTIONS
Order this document by UC3844/D
D SUFFIX
PLASTIC PACKAGE
CASE 751A
(SO–14)
N SUFFIX
PLASTIC PACKAGE
CASE 626
1
8
1
14
(Top V iew)
Vref
(Top V iew)
Compensation
V oltage Feedback
Current Sense
RT/CT
Vref
VCC
Output
Gnd
1
2
3
45
6
7
8
Compensation
NC
V oltage Feedback
NC
Current Sense
NC
RT/CT
NC
VCC
VC
Output
Gnd
Power Ground
1
2
3
4
5
6
7
9
8
10
11
12
13
14
UC3844N
UC3845N
Plastic
Plastic
UC2844D
UC2845D
SO–14
SO–14
UC2844N
UC2845N
Plastic
Plastic
1
MOTOROLA ANALOG IC DEVICE DATA
The UC3844, UC3845 series are high performance fixed frequency
current mode controllers. They are specifically designed for Off–Line and
dc–to–dc converter applications offering the designer a cost effective
solution with minimal external components. These integrated circuits feature
an oscillator, a temperature compensated reference, high gain error
amplifier, current sensing comparator, and a high current totem pole output
ideally suited for driving a power MOSFET.
Also included are protective features consisting of input and reference
undervoltage lockouts each with hysteresis, cycle–by–cycle current limiting,
a latch for single pulse metering, and a flip–flop which blanks the output off
every other oscillator cycle, allowing output deadtimes to be programmed for
50% to 70%.
These devices are available in an 8–pin dual–in–line plastic package as
well as the 14–pin plastic surface mount (SO–14). The SO–14 package has
separate power and ground pins for the totem pole output stage.
The UCX844 has UVLO thresholds of 16 V (on) and 10 V (off), ideally
suited for off–line converters. The UCX845 is tailored for lower voltage
applications having UVLO thresholds of 8.5 V (on) and 7.6 V (off).
•Current Mode Operation to 500 kHz Output Switching Frequency
•Output Deadtime Adjustable from 50% to 70%
•Automatic Feed Forward Compensation
•Latching PWM for Cycle–By–Cycle Current Limiting
•Internally Trimmed Reference with Undervoltage Lockout
•High Current Totem Pole Output
•Input Undervoltage Lockout with Hysteresis
•Low Startup and Operating Current
•Direct Interface with Motorola SENSEFET Products
Simplified Block Diagram
5.0V
Reference
Flip
Flop
&
Latching
PWM
VCC
Undervoltage
Lockout
Oscillator
Error
Amplifier
7(12)
VC7(11)
Output
6(10)
PWR GND
5(8)
3(5)
Current
Sense
Vref
8(14)
4(7)
2(3)
1(1)
Gnd 5(9)
RTCT
Voltage
Feedback
R
R
+
–
Vref
Undervoltage
Lockout
Output
Comp.
Pin numbers in parenthesis are for the D suffix SO–14 package.
VCC
Motorola, Inc. 1996 Rev 1
UC3844, 45 UC2844, 45
2 MOTOROLA ANALOG IC DEVICE DATA
MAXIMUM RATINGS
Rating Symbol Value Unit
Total Power Supply and Zener Current (ICC + IZ)30 mA
Output Current, Source or Sink (Note 1) IO1.0 A
Output Energy (Capacitive Load per Cycle) W 5.0 µJ
Current Sense and Voltage Feedback Inputs Vin – 0.3 to + 5.5 V
Error Amp Output Sink Current IO10 mA
Power Dissipation and Thermal Characteristics
D Suffix, Plastic Package, Case 751A
Maximum Power Dissipation @ TA = 25°C
Thermal Resistance Junction–to–Air
N Suffix, Plastic Package, Case 626
Maximum Power Dissipation @ TA = 25°C
Thermal Resistance Junction–to–Air
PD
RθJA
PD
RθJA
862
145
1.25
100
mW
°C/W
W
°C/W
Operating Junction Temperature TJ+ 150 °C
Operating Ambient Temperature
UC3844, UC3845
UC2844, UC2845
TA0 to + 70
– 25 to + 85
°C
Storage Temperature Range Tstg – 65 to + 150 °C
ELECTRICAL CHARACTERISTICS (VCC = 15 V, [Note 2], RT = 10 k, CT = 3.3 nF, TA = Tlow to Thigh [Note 3],
unless otherwise noted.)
UC284X UC384X
Characteristics Symbol Min Typ Max Min Typ Max Unit
REFERENCE SECTION
Reference Output Voltage (IO = 1.0 mA, TJ = 25°C) Vref 4.95 5.0 5.05 4.9 5.0 5.1 V
Line Regulation (VCC = 12 V to 25 V) Regline –2.0 20 – 2.0 20 mV
Load Regulation (IO = 1.0 mA to 20 mA) Regload –3.0 25 – 3.0 25 mV
Temperature Stability TS– 0.2 – – 0.2 – mV/°C
Total Output Variation over Line, Load, Temperature V ref 4.9 – 5.1 4.82 – 5.18 V
Output Noise Voltage (f = 10 Hz to kHz, TJ = 25°C) Vn– 50 – – 50 – µV
Long Term Stability (TA = 125°C for 1000 Hours) S – 5.0 – – 5.0 – mV
Output Short Circuit Current ISC – 30 – 85 – 180 – 30 – 85 – 180 mA
OSCILLATOR SECTION
Frequency
TJ = 25°C
TA = Tlow to Thigh
fosc 47
46 52
–57
60 47
46 52
–57
60
kHz
Frequency Change with Voltage (VCC = 12 V to 25 V) ∆fosc/∆V–0.2 1.0 – 0.2 1.0 %
Frequency Change with Temperature
TA = Tlow to Thigh ∆fosc/∆T–5.0 – – 5.0 – %
Oscillator V oltage Swing (Peak–to–Peak) Vosc –1.6 – – 1.6 – V
Discharge Current (Vosc = 2.0 V, TJ = 25°C) Idischg –10.8 – – 10.8 – mA
NOTES: 1.Maximum Package power dissipation limits must be observed.
2.Adjust VCC above the Startup threshold before setting to 15 V.
3.Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible
Tlow =–20°C for UC3844, UC3845 Thigh = +70°C for UC3844, UC3845
Tlow =–25°C for UC2844, UC2845 Thigh =+85°C for UC2844, UC2845
UC3844, 45 UC2844, 45
3
MOTOROLA ANALOG IC DEVICE DATA
ELECTRICAL CHARACTERISTICS (VCC = 15 V, [Note 2], RT = 10 k, CT = 3.3 nF, TA = Tlow to Thigh [Note 3],
unless otherwise noted,)
UC284X UC384X
Characteristics Symbol Min Typ Max Min Typ Max Unit
ERROR AMPLIFIER SECTION
Voltage Feedback Input (VO = 2.5 V) VFB 2.45 2.5 2.55 2.42 2.5 2.58 V
Input Bias Current (VFB = 2.7 V) IIB ––0.1 –1.0 – –0.1 –2.0 µA
Open Loop Voltage Gain (VO = 2.0 V to 4.0 V) AVOL 65 90 – 65 90 – dB
Unity Gain Bandwidth (TJ = 25°C) BW 0.7 1.0 – 0.7 1.0 – MHz
Power Supply Rejection Ratio (VCC = 12 V to 25 V) PSRR 60 70 – 60 70 – dB
Output Current
Sink (VO = 1.1 V, VFB = 2.7 V)
Source (VO = 5.0 V, VFB = 2.3 V) ISink
ISource 2.0
–0.5 12
–1.0 –
–2.0
–0.5 12
–1.0 –
–
mA
Output Voltage Swing
High State (RL = 15 k to ground, VFB = 2.3 V)
Low State (RL = 15 k to Vref, VFB = 2.7 V) VOH
VOL 5.0
–6.2
0.8 –
1.1 5.0
–6.2
0.8 –
1.1
V
CURRENT SENSE SECTION
Current Sense Input Voltage Gain (Notes 4 & 5) AV2.85 3.0 3.15 2.85 3.0 3.15 V/V
Maximum Current Sense Input Threshold (Note 4) Vth 0.9 1.0 1.1 0.9 1.0 1.1 V
Power Supply Rejection Ratio
VCC = 12 V to 25 V (Note 4) PSRR – 70 – – 70 – dB
Input Bias Current IIB ––2.0 –10 – –2.0 –10 µA
Propagation Delay (Current Sense Input to Output) tPLH(IN/OUT) –150 300 – 150 300 ns
OUTPUT SECTION
Output Voltage
Low State (ISink = 20 mA)
(ISink = 200 mA)
High State (ISink = 20 mA)
(ISink = 200 mA)
VOL
VOH
–
–
12
12
0.1
1.6
13.5
13.4
0.4
2.2
–
–
–
–
13
12
0.1
1.6
13.5
13.4
0.4
2.2
–
–
V
Output Voltage with UVLO Activated
VCC = 6.0 V, ISink = 1.0 mA VOL(UVLO) – 0.1 1.1 – 0.1 1.1 V
Output Voltage Rise Time (CL = 1.0 nF, TJ = 25°C) tr– 50 150 – 50 150 ns
Output Voltage Fall Time (CL = 1.0 nF, TJ = 25°C) tf– 50 150 – 50 150 ns
UNDERVOLTAGE LOCKOUT SECTION
Startup Threshold
UCX844
UCX845
Vth 15
7.8 16
8.4 17
9.0 14.5
7.8 16
8.4 17.5
9.0
V
Minimum Operating Voltage After T urn–On
UCX844
UCX845
VCC(min) 9.0
7.0 10
7.6 11
8.2 8.5
7.0 10
7.6 11.5
8.2
V
PWM SECTION
Duty Cycle
Maximum
Minimum DCmax
DCmin 46
–48
–50
047
–48
–50
0
%
TOTAL DEVICE
Power Supply Current (Note 2)
Startup:
(VCC = 6.5 V for UCX845A,
(VCC 14 V for UCX844) Operating
ICC
–
–0.5
12 1.0
17 –
–0.5
12 1.0
17
mA
Power Supply Zener Voltage (ICC = 25 mA) VZ30 36 – 30 36 – V
NOTES: 2.Adjust VCC above the Startup threshold before setting to 15 V.
3.Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible
Tlow =–20°C for UC3844, UC3845 Thigh = +70°C for UC3844, UC3845
Tlow =–25°C for UC2844, UC2845 Thigh =+85°C for UC2844, UC2845
4.This parameter is measured at the latch trip point with VFB = 0 V.
5.Comparator gain is defined as: AV∆V Output Compensation
∆V Current Sense Input
UC3844, 45 UC2844, 45
4 MOTOROLA ANALOG IC DEVICE DATA
RT
Ω
, TIMING RESISTOR (k )
Figure 1. Timing Resistor versus
Oscillator Frequency Figure 2. Output Deadtime versus
Oscillator Frequency
Figure 3. Error Amp Small Signal
Transient Response Figure 4. Error Amp Large Signal
Transient Response
0.5
µ
s/DIV
20 mV/DIV
VCC = 15 V
AV = –1.0
TA = 25
°
C
VCC = 15 V
AV = –1.0
TA = 25
°
C
1.0
µ
s/DIV
200 mV/DIV
10 k 20 k 50 k 100 k 200 k 500 k 1.0 M
fosc, OSCILLATOR FREQUENCY (Hz)
VCC = 15 V
TA = 25
°
C
10 k 20 k 50 k 100 k 200 k 500 k 1.0 M
fosc, OSCILLATOR FREQUENCY (Hz)
% DT, PERCENT OUTPUT DEADTIME
Figure 5. Error Amp Open Loop Gain and
Phase versus Frequency Figure 6. Current Sense Input Threshold
versus Error Amp Output Voltage
NOTE: Output switches
at one–half the oscillator
frequency.
CT = 10 nF
5.0 nF
2.0 nF
1.0 nF
100 pF
500 pF 200 pF
2.55 V
2.5 V
2.45 V
2.5 V
3.0 V
2.0 V
100
50
20
10
5.0
2.0
1.0
75
70
65
60
55
50
–20
AVOL, OPEN LOOP VOLTAGE GAIN (dB)
10 M10 f, FREQUENCY (Hz)
Gain
Phase
VCC = 15 V
VO = 2.0 V to 4.0 V
RL = 100 K
TA = 25
°
C
0
30
60
90
120
150
180
100 1.0 k 10 k 100 k 1.0 M
0
20
40
60
80
100
, EXCESS PHASE (DEGREES)
φ
0VO, ERROR AMP OUTPUT VOLTAGE (V)
0
, CURRENT SENSE INPUT THRESHOLD (V)
Vth
0.2
0.4
0.6
0.8
1.0
1.2
2.0 4.0 6.0 8.0
VCC = 15 V
TA = 25
°
C
TA = –55
°
C
TA = 125
°
C
UC3844, 45 UC2844, 45
5
MOTOROLA ANALOG IC DEVICE DATA
Figure 7. Reference Voltage Change
versus Source Current Figure 8. Reference Short Circuit Current
versus Temperature
Figure 9. Reference Load Regulation Figure 10. Reference Line Regulation
∆
, OUTPUT VOLTAGE CHANGE (2.0 mV/DIV)
O
2.0 ms/DIV
V
∆
, OUTPUT VOLTAGE CHANGE (2.0 mV/DIV)
O
2.0 ms/DIV
V
VCC = 12 V to 25 V
TA = 25
°
C
∆
, REFERENCE VOLTAGE CHANGE (mV)
ref
0 20 40 60 80 100 120
Iref, REFERENCE SOURCE CURRENT (mA)
V
VCC = 15 V
TA = –55
°
C
TA = 25
°
C
TA = 125
°
C
, REFERENCE SHORT CIRCUIT CURRENT (mA)
SC
–55 –25 0 25 50 75 100 125
TA, AMBIENT TEMPERATURE (
°
C)
VCC = 15 V
RL
≤
0.1
Ω
I
Figure 11. Output Saturation Voltage
versus Load Current Figure 12. Output Waveform
50 ns/DIV
VCC = 15 V
CL = 1.0 nF
TA = 25
°
C
8006004002000 IO, OUTPUT LOAD CURRENT (mA)
, OUTPUT SATURATION VOLTAGE (V)
sat
V
VCC
TA = 25
°
C
TA = –55
°
C
Sink Saturation
(Load to VCC)Gnd
Source Saturation
(Load to Ground)
TA = –55
°
C
VCC = 15 V
80
µ
s Pulsed Load
120 Hz Rate
TA = 25
°
C
VCC = 15 V
IO = 1.0 mA to 20 mA
TA = 25
°
C
0
–4.0
–8.0
–12
–16
–20
–24
110
90
70
50
90%
10%
0
1.0
2.0
3.0
–2.0
–1.0
0
UC3844, 45 UC2844, 45
6 MOTOROLA ANALOG IC DEVICE DATA
Figure 13. Output Cross Conduction Figure 14. Supply Current versus
Supply Voltage
100 ns/DIV
VCC = 30 V
CL = 15 pF
TA = 25
°
C
, SUPPLY CURRENT
100 mA/DIV 20 V/DIV
I, OUTPUT VOLTAGEV
CC O
25
20
15
10
5
0010203040
, SUPPLY CURRENT (mA)
CC
VCC, SUPPLY VOLTAGE (V)
I
RT = 10 k
CT = 3.3 nF
VFB = 0 V
ISense = 0 V
TA = 25
°
C
UCX845
UCX844
PIN FUNCTION DESCRIPTION
Pin
Fi
Dii
8–Pin 14–Pin Function Description
1 1 Compensation This pin is Error Amplifier output and is made available for loop compensation.
2 3 Voltage
Feedback This is the inverting input of the Error Amplifier. It is normally connected to the switching power
supply output through a resistor divider.
3 5 Current Sense A voltage proportional to inductor current is connected to this input. The PWM uses this
information to terminate the output switch conduction.
4 7 RT/CTThe Oscillator frequency and maximum Output duty cycle are programmed by connecting
resistor RT to Vref and capacitor CT to ground. Operation to 1.0 MHz is possible.
5 – Gnd This pin is combined control circuitry and power ground (8–pin package only).
6 10 Output This output directly drives the gate of a power MOSFET. Peak currents up to 1.0 A are sourced
and sunk by this pin. The output switches at one–half the oscillator frequency.
7 12 VCC This pin is the positive supply of the control IC.
8 14 Vref This is the reference output. It provides charging current for capacitor CT through resistor RT.
– 8 Power Ground This pin is a separate power ground return (14–pin package only) that is connected back to the
power source. It is used to reduce the effects of switching transient noise on the control circuitry.
–11 VCThe Output high state (VOH) is set by the voltage applied to this pin (14–pin package only). With
a separate power source connection, it can reduce the effects of switching transient noise on the
control circuitry.
– 9 Gnd This pin is the control circuitry ground return (14–pin package only) and is connected to back to
the power source ground.
– 2,4,6,13 NC No connection (14–pin package only). These pins are not internally connected.
UC3844, 45 UC2844, 45
7
MOTOROLA ANALOG IC DEVICE DATA
OPERATING DESCRIPTION
The UC3844, UC3845 series are high performance, fixed
frequency, current mode controllers. They are specifically
designed for Off–Line and dc–to–dc converter applications
offering the designer a cost effective solution with minimal
external components. A representative block diagram is
shown in Figure 15.
Oscillator
The oscillator frequency is programmed by the values
selected for the timing components RT and CT. Capacitor CT
is charged from the 5.0 V reference through resistor RT to
approximately 2.8 V and discharged to 1.2 V by an internal
current sink. During the discharge of CT, the oscillator
generates an internal blanking pulse that holds the center
input of the NOR gate high. This causes the Output to be in a
low state, thus producing a controlled amount of output
deadtime. An internal flip–flop has been incorporated in the
UCX844/5 which blanks the output off every other clock cycle
by holding one of the inputs of the NOR gate high. This in
combination with the CT discharge period yields output
deadtimes programmable from 50% to 70%. Figure 1 shows
RT versus Oscillator Frequency and figure 2, Output
Deadtime versus Frequency, both for given values of CT.
Note that many values of RT and CT will give the same
oscillator frequency but only one combination will yield a
specific output deadtime at a given frequency.
In many noise sensitive applications it may be desirable to
frequency–lock the converter to an external system clock.
This can be accomplished by applying a clock signal to the
circuit shown in Figure 17. For reliable locking, the
free–running oscillator frequency should be set about 10%
less than the clock frequency. A method for multi unit
synchronization is shown in Figure 18. By tailoring the clock
waveform, accurate Output duty cycle clamping can be
achieved to realize output deadtimes of greater than 70%
Error Amplifier
A fully compensated Error Amplifier with access to the
inverting input and output is provided. It features a typical dc
voltage gain of 90 dB, and a unity gain bandwidth of 1.0 MHz
with 57 degrees of phase margin (Figure 5). The noninverting
input is internally biased at 2.5 V and is not pinned out. The
converter output voltage is typically divided down and
monitored by the inverting input. The maximum input bias
current is –2.0 µA which can cause an output voltage error
that is equal to the product of the input bias current and the
equivalent input divider source resistance.
The Error Amp Output (Pin 1) is provide for external loop
compensation (Figure 28). The output voltage is offset by two
diode drops (≈ 1.4 V) and divided by three before it connects
to the inverting input of the Current Sense Comparator. This
guarantees that no drive pulses appear at the Output (Pin 6)
when Pin 1 is at its lowest state (VOL). This occurs when the
power supply is operating and the load is removed, or at the
beginning of a soft–start interval (Figures 20, 21). The Error
Amp minimum feedback resistance is limited by the
amplifier’s source current (0.5 mA) and the required output
voltage (VOH) to reach the comparator’s 1.0 V clamp level:
Rf(min) ≈3.0 (1.0 V) + 1.4 V
0.5 mA = 8800 Ω
Current Sense Comparator and PWM Latch
The UC3844, UC3845 operate as a current mode
controller, whereby output switch conduction is initiated by
the oscillator and terminated when the peak inductor current
reaches the threshold level established by the Error Amplifier
Output/Compensation (Pin1). Thus the error signal controls
the inductor current on a cycle–by–cycle basis. The current
Sense Comparator PWM Latch configuration used ensures
that only a single pulse appears at the Output during any
given oscillator cycle. The inductor current is converted to a
voltage by inserting the ground referenced sense resistor RS
in series with the source of output switch Q1. This voltage is
monitored by the Current Sense Input (Pin 3) and compared
a level derived from the Error Amp Output. The peak inductor
current under normal operating conditions is controlled by the
voltage at pin 1 where:
Ipk = V(Pin 1) – 1.4 V
3 RS
Abnormal operating conditions occur when the power
supply output is overloaded or if output voltage sensing is
lost. Under these conditions, the Current Sense Comparator
threshold will be internally clamped to 1.0 V. Therefore the
maximum peak switch current is:
Ipk(max) = 1.0 V
RS
When designing a high power switching regulator it
becomes desirable to reduce the internal clamp voltage in
order to keep the power dissipation of RS to a reasonable
level. A simple method to adjust this voltage is shown in
Figure 19. The two external diodes are used to compensate
the internal diodes yielding a constant clamp voltage over
temperature. Erratic operation due to noise pickup can result
if there is an excessive reduction of the Ipk(max) clamp
voltage.
A narrow spike on the leading edge of the current
waveform can usually be observed and may cause the power
supply to exhibit an instability when the output is lightly
loaded. This spike is due to the power transformer
interwinding capacitance and output rectifier recovery time.
The addition of an RC filter on the Current Sense Input with a
time constant that approximates the spike duration will
usually eliminate the instability; refer to Figure 23.
UC3844, 45 UC2844, 45
8 MOTOROLA ANALOG IC DEVICE DATA
+
–
Sink Only
Positive True Logic
=
RS
+
Internal
Bias
Reference
Regulator
Oscillator
S
RQ
–Vref
UVLO
3.6V
36V
VCC 7(12)
Q1
Vin
VCC
VC
7(11)
6(10)
5(8)
3(5)
+
1.0mA
Error
Amplifier
1(1)
2(3)
4(7)
8(14)
5(9)Gnd
Output
Compensation
Voltage Feedback
Input
RT
CT
Vref
–
–
PWM
Latch
Current Sense
Comparator
R
R
Power Ground
Current Sense Input
2R R1.0V
Pin numbers in parenthesis are for the D suffix SO–14 package.
QT
+
–
+
+
–
+
–
+
VCC
UVLO
Output
2.5V
Figure 15. Representative Block Diagram
Output/
Compensation
Current Sense
Input
Latch
‘‘Reset’ ’ Input
Output
Capacitor CT
Latch
‘‘Set’ ’ Input
Large RT/Small CTSmall R T/Large CT
Figure 16. Timing Diagram
UC3844, 45 UC2844, 45
9
MOTOROLA ANALOG IC DEVICE DATA
Undervoltage Lockout
Two undervoltage lockout comparators have been
incorporated to guartantee that the IC is fully functional before
the output stage is enabled. The positive power supply
terminal (VCC and the reference output (Vref) are each
monitored by separate comparators. Each has built–in
hysteresis to prevent erratic output behavior as their
respective thresholds are crossed. The VCC comparator
upper and lower thresholds are 16 V/10 V for the UCX844,
and 8.4 V/7.6 V for the UCX845. The V ref comparator upper
and lower thresholds are 3.6 V/3/4 V. The large hysteresis
and low startup current of the UCX844 makes it ideally suited
in off–line converter applications where efficient bootstrap
startup techniques later required (Figure 29). The UCX845 is
intended for lower voltage dc–to–dc converter applications. A
36 V zener is connected as a shunt regulator from VCC to
ground. Its purpose is to protect the IC from excessive
voltage that can occur during system startup. The minimum
operating voltage for the UCX844 is 11 V and 8.2 V for the
UCX845.
Output
These devices contain a single totem pole output stage
that was specifically designed for direct drive of power
MOSFETs. It is capable of up to ± 1.0 A peak drive current
and has a typical rise and fall time of 50 ns with a 1.0 nF load.
Additional internal circuitry has been added to keep the
Output in a sinking mode whenever and undervoltage lockout
is active. This characteristic eliminates the need for an
external pull–down resistor.
The SO–14 surface mount package provides separate
pins for VC (output supply) and Power Ground. Proper
implementation will significantly reduce the level of switching
transient noise imposed on the control circuitry. This
becomes particularly useful when reducing the Ipk(max) clamp
level. The separate VC supply input allows the designer
added flexibility in tailoring the drive voltage independent of
VCC. A zener clamp is typically connected to this input when
driving power MOSFETs in systems where VCC is greater the
20 V. Figure 22 shows proper power and control ground
connections in a current sensing power MOSFET
application.
Reference
The 5.0 V bandgap reference is trimmed to ± 1.0%
tolerance at TJ = 25°C on the UC284X, and ± 2.0% on the
UC384X. Its primary purpose is to supply charging current to
the oscillator timing capacitor . The reference has short circuit
protection and is capable of providing in excess of 20 mA for
powering additional control system circuitry.
Design Considerations
Do not attempt to construct the converter on
wire–wrap or plug–in prototype boards. High frequency
circuit layout techniques are imperative to prevent pulsewidth
jitter. This is usually caused by excessive noise pick–up
imposed on the Current Sense or Voltage Feedback inputs.
Noise immunity can be improved by lowering circuit
impedances at these points. The printed circuit layout should
contain a ground plane with low–current signal and
high–current switch and output grounds returning on
separate paths back to the input filter capacitor. Ceramic
bypass capacitors (0.1 µF) connected directly to VCC, VC,
and V ref may be required depending upon circuit layout. This
provides a low impedance path for filtering the high frequency
noise. All high current loops should be kept as short as
possible using heavy copper runs to minimize radiated EMI.
The Error Amp compensation circuitry and the converter
output voltage divider should be located close to the IC and
as far as possible from the power switch and other noise
generating components.
Figure 17. External Clock Synchronization Figure 18. External Duty Cycle Clamp and
Multi–Unit Synchronization
The diode clamp is required if the Sync amplitude is large enough to
cause the bottom side of CT to go more than 300 mV below ground.
External
Sync
Input
47
5(9)
R
RBias
Osc
Vref
RT
8(14)
4(7)
2(3)
1(1)
0.01 CT
2RR
EA
+
–
+
5(9)
R
RBias
Osc
8(14)
4(7)
2(3)
1(1)
2RR
EA
+
–
+
7
5.0k
3
8
6
5
1
C
R
S
MC1455
2
RA
+
–
+
–
4
Q
5.0k
5.0k
RB
T o Additional
UCX84XA’s
f = 1.44
(RA + 2RB)C Dmax = RB
RA + 2RB
UC3844, 45 UC2844, 45
10 MOTOROLA ANALOG IC DEVICE DATA
Figure 19. Adjustable Reduction of Clamp Level Figure 20. Soft–Start Circuit
Figure 21. Adjustable Buffered Reduction of
Clamp Level with Soft–Start Figure 22. Current Sensing Power MOSFET
Virtually lossless current sensing can be achieved with the implement of a SENSEFET
power switch. For proper operation during over current conditions, a reduction of the
Ipk(max) clamp level must be implemented. Refer to Figures 19 and 21.
5(9)
R
RBias
Osc
8(14)
4(7)
2(3)
1(1)
2RR
EA
+
–
+
Q1
RS
3(5)
5(8)
1.0V
–
R
SQ
Comp/Latch
5.0Vref
VClamp
Vin
VCC
7(11)
6(10)
–
+
+
–
+
–+
7(12)
+–
R1 R2
R2
VClamp 1.67
+ 1
+ 0.33 x 10–3 Ipk(max)
≈
VClamp
RS
Where: 0
≤
VClamp
≤
1.0 V
R2
R1
1.0mA
R1
R1 + R2
5(9)
R
RBias
Osc
8(14)
4(7)
2(3)
1(1)
2RR
EA
+
–
+
1.0V
–
R
SQ
5.0Vref
–
+
+
–
+
C
tSoft–Start
3600C in
µ
F
1.0mA
5(9)
R
RBias
Osc
8(14)
4(7)
2(3)
1(1)
2RR
EA
+
–
+
Q1
RS
3(5)
5(8)
1.0V
–
R
SQ
Comp/Latch
5.0Vref
VClamp
Vin
VCC
7(11)
6(10)
–
+
+
–
+
–+
7(12)
+–
MPSA63
R1
R2
C
tSoftstart = – In 1 – VCR1 R2
C
R2
VClamp 1.67
+ 1
Ipk(max)
≈
VClamp
RSWhere: 0
≤
VClamp
≤
1.0 V
1.0mA
R1
3VClamp R1 + R2
RS
1/4 W
(5)
(8)
–
R
SQ
Comp/Latch
5.0Vref
Vin
VCC
(11)
(10)
–
+
+
–
+
–+
(12)
+–
Power Ground
To Input Source
Return
VPin 5
≈
If: SENSEFET = MTP10N10M
RS = 200
Then: Vpin 5 = 0.075 Ipk
SENSEFET
RS Ipk rDS(on)
M
G
D
S
K
Control CIrcuitry
Ground:
To Pin (9)
rDM(on) + RS
+ 0.33 x 10–3 R1 R2
R1 + R2
Figure 23. Current W aveform Spike Suppression
The addition of the RC filter will eliminate
instability caused by the leading edge spike on
the current waveform.
Q1
RS
3(5)
5(8)
–
R
SQ
Comp/Latch
5.0Vref
Vin
VCC
7(11)
6(10)
–
+
+
–
+
–+
7(12)
+–
R
C
T
1.0M
T
TT
T
UC3844, 45 UC2844, 45
11
MOTOROLA ANALOG IC DEVICE DATA
The MCR101 SCR must be selected for a holding of less than 0.5 mA at
TA(min). The simple two transistor circuit can be used in place of the SCR as
shown. All resistors are 10 k.
Figure 24. MOSFET Parasitic Oscillations Figure 25. Bipolar T ransistor Drive
Figure 26. Isolated MOSFET Drive Figure 27. Latched Shutdown
Figure 28. Error Amplifier Compensation
The totem–pole output can furnish negative base current for enhanced
transistor turn–of f, with the addition of capacitor C1.
Error Amp compensation circuit for stabilizing any current–mode topology except
for boost and flyback converters operating with continuous inductor current. Error Amp compensation circuit for stabilizing current–mode boost and flyback
topologies operating with continuous inductor current.
Series gate resistor Rg will damp any high frequency parasitic oscillations
caused by the MOSFET input capacitance and any series wiring inductance
in the gate–source circuit.
Q1
RS
3(5)
5(8)
–
R
SQ
Comp/Latch
5.0Vref
Vin
VCC
7(11)
6(10)
–
+
+
–
+
–+
7(12)
+–
RgQ1
RS
3(5)
5(8)
Vin
6(1)
C1
IB
+
0
–
Base Charge
Removal
ÉÉ
É
É
É
ÉÉ
ÉÉ
Q1
3(5)
5(8)
–
R
SQ
Comp/Latch
5.0Vref
Vin
VCC
7(11)
6(10)
–
+
+
–
+
–+
7(12)
+–
Np
R
CRSNS
Isolation
Boundary
VGS W aveforms
+
0
–
+
0
–
Ipk = V(pin 1) – 1.4
3 RS
NP
NS
50% DC 25% DC
5(9)
R
RBias
Osc
8(14)
4(7)
2(3)
1(1)
2RR
EA
+
–
+1.0mA
2N
3903
2N
3905
MCR
101
5(9)
2(3)
1(1)
2RR
EA
+
–
+1.0mA
CIRf
Ri
Rd
From VO2.5V
5(9)
2(3)
1(1)
2RR
EA
+
–
+1.0mA
CpCIRf
From VO
Rp
Rd
Ri
2.5V
T
Rf
≥
8.8 k
T
Figure 29. 27 Watt Off–Line Flyback Regulator
T1 – Primary: 45 T urns # 26 AWG
T1 – Secondary
±
12 V : 9 Turns # 30 AWG
T1 – (2 strands) Bifiliar Wound
T1 – Secondary 5.0 V : 4 Turns (six strands)
T1 – #26 Hexfiliar Wound
T1 – Secondary Feedback: 10 T urns #30 A WG
T1 – (2 strands) Bifiliar Wound
T1 – Core: Ferroxcube EC35–3C8
T1 – Bobbin: Ferroxcube EC35PCB1
T1 – Gap
≈
0.01” for a primary inductance of 1.0 mH
L1 – 15
µ
H at 5.0 A, Coilcraft Z7156.
L2, L3 – 25
µ
H at 1.0 A, Coilcraft Z7157.
Comp/Latch
S
RQ
1N4935 1N4935
5.0Vref
Bias
Osc
++ 47
100
EA
+
+
7(12)
L1
5.0V/4.0A
2200 1000 +
MUR110
MBR1635
1000
1000 10
++
+L2
5.0V RTN
12V/0.3A
1N4937
L3
MUR110
±
12V RTN
–12V/0.3A
T1
1.0k
470pF
3(5)
5(8)
6(10)
7(11) 22
Ω
1N4937
2.7k
3300pF
4.7k
56k
250
+
115Vac
4.7
Ω
MDA
202
68
5(9)
+
1(1)
2(3)
4(7)
33k
0.01
1.0nF
18k
4.7k
MTP
4N50
8(14)
10
+
+
680pF
0.5
Ω
150k
100pF
++
+
+
–
–
–
–
1N5819
T
UC3844, 45 UC2844, 45
12 MOTOROLA ANALOG IC DEVICE DATA
Test Conditions Results
Line Regulation: 5.0 V
± 12 V Vin = 95 Vac to 130 Vac ∆ = 50 mV or ± 0.5%
∆ = 24 mV or ± 0.1%
Load Regulation: 5.0 V
± 12 V Vin = 115 Vac, Iout = 1.0 A to 4.0 A
Vin = 115 Vac, Iout = 100 mA to 300 mA ∆ = 300 mV or ± 3.0%
∆ = 60 mV or ± 0.25%
Output Ripple: 5.0 V
± 12 V Vin = 115 Vac 40 mVpp
80 mVpp
Efficiency Vin = 115 Vac 70%
All outputs are at nominal load currents, unless otherwise noted.
UC3844, 45 UC2844, 45
13
MOTOROLA ANALOG IC DEVICE DATA
+
–
+
Internal
Bias
Reference
Regulator
Oscillator
S
RQ
–Vref
UVLO
3.6V
34V
7(12)
Vin = 15V
7(11)
6(10)
5(8)
3(5)
+
0.5mA
Error
Amplifier
1(1)
2(3)
4(7)
8(14)
5(9)
10k
1.0nF
–
–
PWM
Latch
Current Sense
Comparator
R
R
2R
R1.0V
The capacitor’s equivalent series resistance must limit the Drive Output current to 1.0 A. An additional series
resistor may be required when using tantalum or other low ESR capacitors. The converter’s output can provide
excellent line and load regulation by connecting the R2/R1 resistor divider as shown.
T
+
+
–
+
–
+
VCC
UVLO
2.5V
UC3845 +47
1N5819
+
15 10 1N5819
Connect to
Pin 2 for
closed loop
operation.
+47
R2
R1
VO
2 (Vin)
VO = 2.5 + 1
R2
R2
Output Load Regulation
(open loop configuration)
IO (mA) VO (V)
0
2
9
18
36
29.9
28.8
28.3
27.4
24.4
Figure 30. Step–Up Charge Pump Converter
+
–
+
Internal
Bias
Reference
Regulator
Oscillator
S
RQ
–Vref
UVLO
3.6V
34V
7(12)
Vin = 15V
7(11)
6(10)
5(8)
3(5)
+
0.5mA
Error
Amplifier
1(1)
2(3)
4(7)
8(14)
5(9)
10k
1.0nF
–
–
PWM
Latch
Current Sense
Comparator
R
R
2R
R1.0V
The capacitor’s equivalent series resistance must limit the Drive Output current to 1.0 A.
An additional series resistor may be required when using tantalum or other low ESR capacitors.
T
+
+
–
+
–
+
VCC
UVLO
2.5V
UC3845 +47
+
15 10 1N5819
+47 VO
– (Vin)
Output Load Regulation
IO (mA) VO (V)
0
2
9
18
32
–14.4
–13.2
–12.5
–11.7
–10.6
1N5819
Figure 31. Voltage–Inverting Charge Pump Converter
UC3844, 45 UC2844, 45
14 MOTOROLA ANALOG IC DEVICE DATA
OUTLINE DIMENSIONS
N SUFFIX
PLASTIC PACKAGE
CASE 626–05
ISSUE K
D SUFFIX
PLASTIC PACKAGE
CASE 751A–03
(SO–14)
ISSUE F
NOTES:
1. DIMENSION L TO CENTER OF LEAD WHEN
FORMED PARALLEL.
2. PACKAGE CONTOUR OPTIONAL (ROUND OR
SQUARE CORNERS).
3. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
14
58
F
NOTE 2 –A–
–B–
–T–
SEATING
PLANE
H
J
GDK
N
C
L
M
M
A
M
0.13 (0.005) B M
T
DIM MIN MAX MIN MAX
INCHESMILLIMETERS
A9.40 10.16 0.370 0.400
B6.10 6.60 0.240 0.260
C3.94 4.45 0.155 0.175
D0.38 0.51 0.015 0.020
F1.02 1.78 0.040 0.070
G2.54 BSC 0.100 BSC
H0.76 1.27 0.030 0.050
J0.20 0.30 0.008 0.012
K2.92 3.43 0.115 0.135
L7.62 BSC 0.300 BSC
M––– 10 ––– 10
N0.76 1.01 0.030 0.040
__
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSIONS A AND B DO NOT INCLUDE
MOLD PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
–A–
–B–
G
P7 PL
14 8
71 M
0.25 (0.010) B M
S
B
M
0.25 (0.010) A S
T
–T–
F
RX 45
SEATING
PLANE
D14 PL K
C
J
M
_
DIM MIN MAX MIN MAX
INCHESMILLIMETERS
A8.55 8.75 0.337 0.344
B3.80 4.00 0.150 0.157
C1.35 1.75 0.054 0.068
D0.35 0.49 0.014 0.019
F0.40 1.25 0.016 0.049
G1.27 BSC 0.050 BSC
J0.19 0.25 0.008 0.009
K0.10 0.25 0.004 0.009
M0 7 0 7
P5.80 6.20 0.228 0.244
R0.25 0.50 0.010 0.019
____
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the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and
specifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motorola
data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”
must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights of
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applications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury
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Opportunity/Af firmative Action Employer .
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UC3844/D
*UC3844/D*
◊
