QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
1
LT1976
DESCRIPTION
Demonstration circuit 481 is a monolithic step-
down DC/DC switching converter featuring the
LT1976. The board is optimized for 3.3V output at
up to 1A load current for an input voltage range of
4V to 60V. Minimum on-time restrictions and 3.3V
output may limit the steady state maximum input
voltage to 42V (up to 47V for some boards) before
pulse skipping occurs.
With its wide input voltage
range, 1.5A internal power switch, 100
µ
A supply
current (at input voltages above 14V), 200kHz
switching frequency and thermally enhanced
package, the LT1976 is a very versatile and pow-
erful IC for DC/DC converters that require com-
pact space, high efficiency at both heavy and light
loads and high input voltage.
The LT1976 200kHz switching frequency allows all
of the components to be small, surface mount de-
vices. Synchronization with an external clock of up
to 700kHz is possible. The current-mode control
topology creates fast transient response and good
loop stability with a minimum number of external
components. The low resistance internal power
switch (0.2
Ω
) achieves high efficiencies of up to
90%. The SHDN pin can be used to program un-
dervoltage lockout or place the part in micropower
shutdown, reducing supply current to less than
1µA by driving the pin low. Burst Mode™ reduces
zero load current to under 100µA at most input
voltages (see Figure 3) while maintaining a regu-
lated output. A power good comparator and a tim-
ing delay can be used for additional system diag-
nostics and sequencing. The soft start function
reduces inrush current at soft start and output
voltage overshoot.
The LT1976 datasheet gives a complete descrip-
tion of the part, operation and applications infor-
mation. The datasheet must be read in conjunc-
tion with this Quick Start Guide for demonstration
circuit 481. In particular, the datasheet section on
‘Thermal Calculations’ is important for estimating
whether a given application’s combination of input
voltage, load current and frequency will cause the
LT1976 to exceed it’s absolute maximum rated
junction temperature. The LT1976 is assembled in
a small 16-pin thermally enhanced package with
exposed pad where proper board layout is essen-
tial for maximum thermal performance. See the
datasheet section ‘Layout Considerations’.
Design files for this circuit board are available.
Call the LTC factory.
Burst Mode is a trademark of Linear Technology Corporation.
Table 1. Typical Performance Summary (T
A
= 25°C)
PARAMETER CONDITION VALUE
Steady State Input Voltage Range V
OUT
= 3.3V, I
OUT
≤
1A 4–42V
Maximum Transient Input Voltage 60V
V
OUT
V
IN
= 4V to 60V, I
OUT
≤
1A 3.3V ± 3%
Maximum Output Current V
OUT
= 3.3V 1A
V
IN
= 12V, I
OUT
= 1A, V
OUT
= 3.3V 34mV
PK–PK
Output Voltage Ripple V
IN
= 42V, I
OUT
= 1A, V
OUT
= 3.3V 42mV
PK-PK
Switching Frequency V
IN
= 4V to 42V, I
OUT
≤
1A 200kHz
V
IN
= 12V, I
OUT
= 1A, V
OUT
= 3.3V 80%
Efficiency V
IN
= 42V, I
OUT
= 1A, V
OUT
= 3.3V 72%
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
2
QUICK START PROCEDURE
Demonstration circuit 481 is easy to set up to
evaluate the performance of the LT1976. Refer to
Figure 1 for proper measurement equipment setup
and follow the procedure below:
NOTE:
Make sure that the input voltage does not
exceed 60V.
NOTE:
The synchronization, shutdown, and power
good functions are optional and their terminals
can be left floating (disconnected) if their functions
are not being used.
NOTE:
Do not hot-plug the input voltage terminal
VIN. The absolute maximum voltage on VIN is
60V and hot-plugging a power supply through wire
leads to the demonstration circuit can cause the
voltage on the extremely low-ESR ceramic input
capacitor to ring to twice its DC value. This is due
to high currents instantaneously generated in the
inductive supply leads from an input voltage step
on the low-ESR ceramic input capacitor. A bulky
higher-ESR capacitor, and an additional inductive
filter can be added to the circuit to dampen hot-
plug transient ringing. See Application Note 88 for
more details. In order to protect the IC, a transient
voltage suppressor diode can be added between
VIN and GND terminals to absorb any high volt-
age transient ringing that may occur due to hot-
plugging.
NOTE:
Connect the power supply (with power off),
load, and meters as shown in Figure 1.
1.
After all connections are made, turn on input
power and verify that the output voltage is 3.3V.
NOTE:
If the output voltage is too low, temporar-
ily disconnect the load to make sure that the
load is not set too high.
2.
Once the proper output voltages are estab-
lished, adjust the load within the operating
range and observe the output voltage regula-
tion, ripple voltage, efficiency and other parame-
ters.
Figure 1. Proper Measurement Equipment Setup
FUNCTIONS & OPTIONS
BURST MODE
Burst Mode operation reduces light load quiescent
current by disabling switching for a number of
switch cycles and reducing supply current briefly
until switching begins again. Bursts of switch
pulses are enough to maintain output voltage
regulation at light load. Figure 3 shows the supply
current versus input voltage. At input voltages
above 12V, zero load supply current is less than
110µA.
Burst Mode extends battery life in applications
with long periods of inactivity. The tradeoff of hav-
ing Burst Mode is creating increased output volt-
age ripple at light load. For loads that require ex-
tremely well-regulated output voltages, additional
filtering may be used on the output to achieve ex-
tremely low output voltage ripple. At higher input
voltages (above 35V
IN
), Burst Mode may occur
below load currents as high as 300mA to 500mA.
OUTPUT VOLTAGE
The components assembled on the board are op-
timized for a wide input voltage range and a 3.3V
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
3
output. The feedback resistors (R2, R3) can be
changed to adjust the output voltage according to
the following equation:
V
OUT
= 1.24
×
(1 + R2/R3)
For output voltages below 3V, the boost pin re-
quires a higher voltage than the output can supply.
An alternate source for boost such as the input
voltage, a bias supply, or an external supply is re-
quired on the boost pin. Please see the datasheet
for details.
For output voltages greater than 5V, the optional
‘blocking’ zener diode D3 can be used to reduce
the boost voltage across C4 to some lower voltage
between 3V and 5V. The diode transfers power
dissipation from inside the LT1976 to the diode on
the demonstration circuit, outside the LT1976, al-
lowing higher ambient temperature operation of
the part. Maintaining boost voltage between 3V
and 5V maximizes efficiency and optimizes control
of the power switch. It is recommended that a
CMHZ5236B zener diode is used in D3 when
V
OUT
= 12V. To properly install D3, the small trace
shorting the anode to the cathode of D3 on the
board must be opened (an Exacto knife works
well) before D3 is soldered to the board. In the
‘Thermal Calculations’ section of the datasheet,
the new value for boost voltage (V
OUT
–V
Z
) should
be accounted for when calculating junction tem-
perature.
P
BOOST
= (V
OUT
-V
Z
)*V
OUT
*(I
OUT
/36)/V
IN
POWER GOOD FEEDBACK OPTION
For systems that rely upon having a well-regulated
power source or follow a particular power-up se-
quence, the LT1976 provides a power good flag
with timed delay programmed by C8 when the
power good feedback pin (PGFB) exceeds 90% of
V
REF
(1.25V). R10 (0 ohm short) ties PGFB and
the feedback pin (FB) together. Therefore, the
power good (PG) pin returns a ‘good’ signal when
the output voltage has reached 90% of its final
value. Figure 11 and Figure 12 show the power
good logic output turning on after a programmed
delay during startup.
The power good feedback pin can also be tied to
the input voltage, an external source, or a resistor
divider on any of these sources. Removing R10
breaks the connection between PGFB and FB.
The Power Good Feedback (PGFB Option) termi-
nal is optional and is not stuffed on the board. The
power good terminal node can be connected to
the power good feedback (PGFB) pin by placing a
0
Ω
resistor in R11. The PGFB Option should be
used when Power Good Feedback is required
from a source other than the feedback pin. Be
sure to remove the connection between PGFB
and FB by removing R10 as mentioned above.
Connect the desired Power Good Feedback
source to the PGFB Option terminal and either
short the terminal to PGFB pin with a 0
Ω
resistor
in R11 or place a resistor divider from PGFB to
GND with R11 and R12.
SHUTDOWN AND UNDERVOLTAGE LOCKOUT
The SHDN pin has a 200k pull-up resistor (R9)
tied to V
IN
. For normal operation, the SHDN
terminal can be left floating. However, connecting
the SHDN terminal to GND will place the IC in mi-
cropower shutdown. If the shutdown function is
not being used, the pull-up resistor can be re-
placed with a 0
Ω
resistor.
For undervoltage lockout, the two-resistor divider
network must be placed between V
IN
and SHDN
and between SHDN and GND. The top resistor
can be placed in R9. The bottom resistor can be
placed to the right of the SHDN terminal (the sol-
der mask may have to be removed.
Please see the data sheet section ‘Shutdown
Function and Undervoltage Lockout’ for more de-
tails.
SOFT START
Soft start removes the inrush current surge and
limits output voltage overshoot by controlling the
output voltage ramp-up rate. A single capacitor,
C7, holds the peak current level clamp low, allow-
ing it to slowly rise upon startup. When a short cir-
cuit, overload, or shutdown condition occurs, the
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
4
soft start capacitor resets to zero and provides soft
start during restart. Switchers that do not have soft
start may transition from zero output to full output
voltage by taking as much current as possible
from the source and casting it into the output ca-
pacitor and load. This surge of current, only re-
stricted by maximum peak switch current levels,
can both drag down a battery source voltage and
cause overshoot in the output voltage.
For the shortest possible startup time, remove the
soft start capacitor from the circuit. Maximum in-
rush current can reach the level of 3A (the maxi-
mum switch current limit). Expect to see a signifi-
cant increase in output voltage overshoot.
Figure 11 and Figure 12 show the soft startup of
DC481 and the limited inrush current during
startup.
SYNCHRONIZATION
The synchronization frequency range for the
LT1976 is 235kHz to 700kHz. Use a logic level
sync signal with a duty cycle between 10% and
90% connected directly to the SYNC pin. Keep in
mind that synchronization at high frequencies may
reduce the effect of slope compensation. High
sync frequencies combined with high duty cycles
(above 50%) may result in unexpected loop insta-
bility.
COMPENSATION
Demonstration Circuit 481 has a frequency com-
pensation network that is optimized for the tanta-
lum output capacitor C5, the wide input voltage
range 4V to 60V (42V steady state), and 3.3V out-
put. Improved loop bandwidth can be achieved for
various output voltages, output capacitors, and
input voltage ranges by adjusting R1, C2, and C1.
A feedforward capacitor (C10) and a resistor (R4
for short circuit feedback pin protection when C10
is used) are located in parallel with R2. Removing
these components from the feedback loop may
result in compromised loop stability. The use of
alternate output capacitors such as ceramics or
PosCaps may require changes to the compensa-
tion components. For more information, see the
‘Frequency Compensation’ section in the Applica-
tions Information in the datasheet, Application
Note 19, or Application Note 76.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
5
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1 10 100 1000
VOUT LOAD CURRENT (mA)
EFFICIENCY (%)
12VIN
42VIN
Figure 2. DC481 Typical Efficiency (T
A
= 25°C, V
OUT
= 3.3V)
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
0 5 10 15 20 25 30 35 40 45 50 55 60
VIN (V)
VIN SUPPLY CURRENT W/ ZERO LOAD (
µA)
Figure 3. DC481 Typical Supply Current (I
OUT
= 0A, T
A
= 25°C, V
OUT
= 3.3V)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
6
Figure 4. DC481 Typical Step Load Response (I
OUT
= 500mA to 1A, V
IN
= 12V, T
A
= 25°C,
V
OUT
= 3.3V) CH2 is V
OUT
(AC) CH4 is I
OUT
(500mA/10mV
Ω)
Ω)Ω)
Ω)
Figure 5. DC481 Typical Step Load Response (I
OUT
= 500mA to 1A, V
IN
= 42V, T
A
= 25°C,
V
OUT
= 3.3V) CH2 is V
OUT
(AC) CH4 is I
OUT
(500mA/10mV
Ω)
Ω)Ω)
Ω)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
7
Figure 6. DC481 Typical Output Voltage Ripple (I
OUT
= 500mA, V
IN
= 5V, V
OUT
= 3.3V, T
A
=
25°C) CH3 is V
OUT
ripple (AC), CH2 is V
SW
Figure 7. DC481 Typical Output Voltage Ripple (I
OUT
= 1A, V
IN
= 12V, V
OUT
= 3.3V, T
A
= 25°C)
CH2 is V
OUT
ripple (AC), CH3 is V
SW
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
8
Figure 8. DC481 Typical Output Voltage Ripple (I
OUT
= 1A, V
IN
= 42V, V
OUT
= 3.3V, T
A
= 25°C)
CH2 is V
OUT
ripple (AC), CH3 is V
SW
Figure 9. DC481 Typical Output Voltage Ripple in Burst Mode (I
OUT
= 100mA, V
IN
= 12V, V
OUT
= 3.3V, T
A
= 25°C) CH1 is V
OUT
ripple (AC), CH2 is V
SW
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
9
Figure 10. DC481 Typical Output Voltage Ripple in Burst Mode (I
OUT
= 100mA, V
IN
= 42V,
V
OUT
= 3.3V, T
A
= 25°C) CH1 is V
OUT
ripple (AC), CH2 is V
SW
Figure 11. DC481 Soft Startup (R
LOAD
= 3.3
Ω
ΩΩ
Ω
, V
IN
= 12V, V
OUT
= 3.3V, T
A
= 25°C) CH2 is V
OUT
,
CH3 is Power Good, CH4 is I
IN
(200mA/10.0mV
Ω)
Ω)Ω)
Ω)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
10
Figure 12. DC481 Soft Startup (R
LOAD
= 7
Ω
ΩΩ
Ω
, V
IN
= 42V, V
OUT
= 3.3V, T
A
= 25°C) CH2 is V
OUT
,
CH3 is Power Good, CH4 is I
IN
(100mA/10.0mV
Ω)
Ω)Ω)
Ω)
DC481A LT1976IFE 20VIN 3.3VOUT 1A
20
30
40
50
60
0 1 2 3 4 5
Time (min)
Figure 13. Five Minute Thermal Profile of DC481
×
××
×
10 (I
OUT
= 1A, V
IN
= 20V, T
A
= 25°C, V
OUT
=
3.3V)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
11
DC481A LT1976IFE 42VIN 3.3VOUT 1A
20
30
40
50
60
0 1 2 3 4 5
Time (min)
Figure 14. Five Minute Thermal Profile of DC481
×
××
×
10 (I
OUT
= 1A, V
IN
= 42V, T
A
= 25°C, V
OUT
=
3.3V)
Figure 15. Bode Plot (Phase and Gain I
OUT
= 1A, V
IN
= 5V, T
A
= 25°C, V
OUT
= 3.3V)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
12
Figure 16. Bode Plot (Phase and Gain I
OUT
= 1A, V
IN
= 12V, T
A
= 25°C, V
OUT
= 3.3V)
Figure 17. Bode Plot (Phase and Gain I
OUT
= 1A, V
IN
= 42V, T
A
= 25°C, V
OUT
= 3.3V)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 481
1.5A, 200KHZ STEP-DOWN CONVERTER WITH BURST MODE
13
A
A
B
B
C
C
D
D
E
E
4 4
3 3
2 2
1 1
4
15
14
7
13
8
9
16
11
12
10
2
6
1
3
5
VIN
SHDN
SYNC
TCAP
PGFB
GND
CSS
PGOOD
VC
FB
BIAS
SW
BOOST
SW
VIN
SW
Linear Technology Corporation
LT1976IFE
Bill Of Material
Item Qty Reference Part Description Manufacture / Part #
1 1 C1 Cap., NPO 330pF 50V 10% AVX 06035A331KAT1A
2 1 C2 Cap., X7R 1500pF 25V 10% AVX 06033C152KAT1A
3 1 C3 Cap., X7R 10uF 50V 20% TDK C5750X7R1H106M
4 3 C4,C7,C8 Cap., X7R 0.10uF 16V 20% Taiyo Yuden EMK107BJ104M
5 1 C5 Cap., Tant. 100uF 6.3V 20% AVX TPSC107M006R075
6 0 C6 (Optional) OPT.
7 0 C9 (Optional) OPT.
8 1 C10 Cap., NPO 47pF 50V 10% AVX 06035A470KAT1A
9 1 D1 Schottky Rect., 2.1Amp\60V IR 10MQ060N
10 1 D2 Diode, 75V/200mW Diodes Inc. 1N4148WS
11 0 D3 (Optional) OPT.
12 1 L1 Inductor, 33uH Sumida CDRH8D28-330NC
13 1 R1 Res., Chip 10k 0.1W 5% AAC CR16-103JM
14 1 R2 Res., Chip 165k 0.1W 1% AAC CR16-1653FM
15 1 R3 Res., Chip 100k 0.1W 1% AAC CR16-1003FM
16 1 R4 Res., Chip 24k 0.1W 5% AAC CR16-243JM
17 1 R8 Res., Chip 100k 0.1W 5% AAC CR16-104JM
18 1 R9 Res., Chip 200k 0.06W 5% AAC CR16-204JM
19 1 R10 Jumper, Chip 0 Ohm 1/16W 1AMP AAC CJ06-000M
20 0 R11,R12 (Optional) OPT.
21 7 TP1-TP7 Turret, Testpoint Mill Max 2501-2
22 0 TP8 (Optional) OPT.
23 1 U1 I.C., Step-Down Reg. Linear Tech. Corp. LT1976IFE
24 1 PRINTED CIRCUIT BOARD DEMO CIRCUIT #481A
25 1 STENCIL STENCIL 481A
