LM2903PWR**CH-ART - TEXAS INSTRUMENTS

April 2005 1 MIC3808/3809

MIC3808/3809 Micrel

MIC3808/3809

Push-Pull PWM Controller

General Description

The MIC3808 and MIC3809 are a family of complementary

output push-pull PWM control ICs that feature high speed and

low power consumption. The MIC3808/9 are ideal for telecom

level (36V to 75V) isolated step down dc/dc conversion

applications where high output current, small size, and high

efficiency are required.

The dual-ended push-pull architecture of the MIC3808/9

allows more efficient utilization of the transformer than single-

ended topologies, allowing smaller size dc/dc solutions.

Additionally, the out-of-phase push-pull topology allows a

higher effective duty cycle, reducing input and output ripple

as well as stress on the external components. The dead-time

between the two outputs is adjustable between 60ns to

200ns, limiting the duty cycle of each output stage to less than

50%.

The MIC3808/9 are built on a low-power, high-speed BiCMOS

process. The 130µA start-up current and 1mA run-current

reduce the size of the start-up circuitry and allow high

efficiency even at light loads. The high-speed internal 4MHz

error amplifier allows MIC3808/9 operation up to 1MHz.

The MIC3808 has a turn-on threshold of 12.5V whereas the

MIC3809 has a lower turn-on threshold of 4.3V. Both devices

are available in SOP-8 and MSOP-8 package options with an

operating range of –40°C to +85°C.

Data sheets and support documentation can be found on

Micrel’s web site at: www.micrel.com.

Features

• Dual output drive stages in push-pull configuration

• Leading edge current-sense blanking

• 130µA typical start-up current

• 1mA typical run current

• Operation to 1MHz

• Internal soft start

• On-chip error amplifier with 4MHz gain bandwidth

product

• On-chip VDD clamping

• Output drive stages capable of 500mA peak source

current, 1A peak sink current

Applications

• High efficiency “brick” power supply modules

• Half bridge converters

• Full bridge converters

• Push-pull converters

• Voltage-fed push-pull converters

• Telecom equipment and power supplies

• Networking power supplies

• Industrial power supplies

• 42V automotive power supplies

• Base stations

Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com

Typical Application

VIN

36V to 75V

MIC3808

VOUT

12V 100W

GND

VDD

OUTA

COMP OUTB

Start-Up

Circuitry

Slope

Comp

Reference

& Isolation

MIC3808/3809 Micrel

MIC3808/3809 2 April 2005

Pin Configuration

1COMP

8 VDD

OUTA

OUTB

GND

SOIC-8 (M)

MSOP-8 (MM)

Pin Description

Pin Number Pin Name Pin Function

1 COMP COMP is the output of the error amplifier and the input of the PWM com-

parator. The error amplifier in the MIC3808 is a true low-output impedance,

4MHz operational amplifier. As such, the COMP pin can both source and

sink current. However, the error amplifier is internally current limited, so that

zero duty cycle can be externally forced by pulling COMP to GND. The

MIC3808 family features built-in full cycle soft start. Soft start is imple-

mented as a clamp on the maximum COMP voltage.

2 FB The inverting input to the error amplifier. For best stability, keep FB lead

length as short as possible and FB stray capacitance as small as possible.

3 CS The input to the PWM, peak current, and overcurrent comparators. The

overcurrent comparator is only intended for fault sensing. Exceeding the

overcurrent threshold will cause a soft start cycle. An internal MOSFET

discharges the current sense filter capacitor to improve dynamic perfor-

mance of the power converter.

4 RC The oscillator programming pin. The MIC3808’s oscillator tracks VDD and

GND internally, so that variations in power supply rails minimally affect

frequency stability. Only two components are required to program the

oscillator, a resistor (tied to the VDD and RC), and a capacitor (tied to the RC

and GND). The approximate oscillator frequency is determined by the simple

formula:

F1.41

OSCILLATOR =

where frequency is in Hertz, resistance in Ohms, and capacitance in Farads.

The recommended range of timing resistors is between 7kΩ and 200kΩ and

range of timing capacitors is between 100pF and 1000pF. Timing resistors

less than 7kΩ should be avoided. For best performance, keep the timing

capacitor lead to GND as short as possible, the timing resistor lead from VDD

as short as possible, and the leads between timing components and RC as

short as possible. Separate ground and VDD traces to the external timing

network are encouraged.

Ordering Information

Part Number

Turn On Turn Off

Standard Lead-Free Threshold Threshold Temperature Range Package

MIC3808BM MIC3808YM 12.5V 8.3V –40°C to +85°C 8-Pin SOIC

MIC3809BM MIC3809YM 4.3V 4.1V –40°C to +85°C 8-Pin SOIC

MIC3808BMM MIC3808YMM 12.5V 8.3V –40°C to +85°C 8-Pin MSOP

MIC3809BMM MIC3809YMM 4.3V 4.1V –40°C to +85°C 8-Pin MSOP

April 2005 3 MIC3808/3809

MIC3808/3809 Micrel

Pin Description

Pin Number Pin Name Pin Function

5 GND Ground

6 OUTB Alternating high current output stages. Both stages are capable of driving the

gate of a power MOSFET. Each stage is capable of 500mA peak source

7 OUTA current, and 1A peak sink current. The output stages switch at half the

oscillator frequency, in a push/pull configuration. When the voltage on the

RC pin is rising, one of the two outputs is high, but during fall time, both

outputs are off. This “dead time” between the two outputs, along with a

slower output rise time than fall time, insures that the two outputs can not be

on at the same time. This dead time is typically 60ns to 200ns and depends

upon the values of the timing capacitor and resistor. The high-current output

drivers consist of MOSFET output devices, which switch from VDD to GND.

Each output stage also provides a very low impedance to overshoot and

undershoot. This means that in many cases, external Schottky clamp diodes

are not required.

8 VDD The power input connection for this device. Although quiescent VDD current

is very low, total supply current will be higher, depending on OUTA and

OUTB current, and the programmed oscillator frequency. Total VDD current

is the sum of quiescent VDD current and the average OUT current. Knowing

the operating frequency and the MOSFET gate charge (Qg), average OUT

current can be calculated from IOUT = Qg × F, where F is frequency. To

prevent noise problems, bypass VDD to GND with a ceramic capacitor as

close to the chip as possible. A 1µF decoupling capacitor is recommended.

MIC3808/3809 Micrel

MIC3808/3809 4 April 2005

Electrical Characteristics

TA = TJ = –40°C to +85°C, VDD = 10V (Note 9), 1µF capacitor from VDD to GND, R = 22KΩ, C = 330pF.

Parameter Condition Min Typ Max Units

Oscillator Section

Oscillator Frequency 180 200 220 kHz

Oscillator Amplitude/VDD Note 4 0.44 0.5 0.56 V/V

Error Amp Section

Input Voltage COMP = 2V 1.95 2 2.05 V

Input Bias Current –1 1 µA

Open Loop Voltage Gain (Guaranteed by design) 60 80 dB

COMP Sink Current FB = 2.2V, COMP = 1V 0.3 2.5 mA

COMP Source Current FB = 1.3V, COMP = 3V, Note 5 –0.15 –0.5 mA

COMP Clamp Voltage VFB = 0V 3.1 3.6 4.0 V

PWM Section

Maximum Duty Cycle Measured at OUTA or OUTB 48 49 50 %

Minimum Duty Cycle COMP = 0V 0%

Current Sense Section

Gain Note 6 (Guaranteed by design) 1.9 2.2 2.5 V/V

Maximum Input Signal Note 7 0.45 0.5 0.55 V

CS to Output Delay COMP = 3V, CS from 0 to 600mV 70 200 ns

CS Source Current –200 nA

CS Sink Current CS = 0.5V, RC = 5.5V, Note 8 510 mA

Over Current Threshold 0.7 0.75 0.8 V

COMP to CS Offset CS = 0V 0.35 0.8 1.2 V

Output Section

OUT Low Level I = 100mA 0.5 1 V

OUT High Level I = –50mA, VDD - OUT 0.5 1 V

Rise Time CL = 1nF 25 60 ns

Fall Time CL = 1nF 25 60 ns

Absolute Maximum Ratings (Note 1)

Supply Voltage (IDD ≤ 10mA) ...................................... +15V

Supply Current ........................................................... 20mA

OUTA/OUTB Source Current (peak).......................... –0.5A

OUTA/OUTB Sink Current (peak) ................................ 1.0A

Comp Pin ......................................................................VDD

Analog Inputs (FB, CS) ........................ –0.3V to VDD +0.3V

NOT TO EXCEED 6V

Junction Temperature .............................. –55°C to +150°C

Storage Temperature ............................... –65°C to +150°C

Lead Temperature (soldering, 10 sec.) ................... +300°C

ESD Rating, Note 3 ...................................................... 2kV

Operating Ratings (Note 2)

VDD Input Voltage (VDD) ........................................ Note 11

Oscillator Frequency (fOSC) ....................... 10kHz to 1MHz

Ambient Temperature (TA) ......................... –40°C to +85°C

Package Thermal Resistance

SOIC-8 (θJA) ...................................................... 160°C/W

MSOP-8 (θJA) .................................................... 206°C/W

April 2005 5 MIC3808/3809

MIC3808/3809 Micrel

Parameter Condition Min Typ Max Units

Undervoltage Lockout Section

Start Threshold MIC3808, Note 9 11.5 12.5 13.5 V

MIC3809 4.1 4.3 4.5 V

Minimum Operating Voltage MIC3808 7.6 8.3 9 V

After Start MIC3809 3.9 4.1 4.3 V

Hysteresis MIC3808 3.5 4.2 5.1 V

MIC3809 0.1 0.2 0.3 V

Soft Start Section

COMP Rise Time FB = 1.8V, Rise from 0.5V to 3V 2.5 20 ms

Overall Section

Startup Current VDD < Start Threshold 130 260 µA

Operating Supply Current FB = 0V, CS = 0V, Notes 9 and 10 12mA

VDD Zener Shunt Voltage IDD = 10mA, Note 12 13 14 15 V

Note 1. Exceeding the absolute maximum rating may damage the device.

Note 2. The device is not guaranteed to function outside its operating rating.

Note 3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.

Note 4. Measured at RC. Signal amplitude tracks VDD.

Note 5. The COMP pin is internally clamped to 3.65V(typ). The COMP pin source current is tested at VCOMP = 3.0V to avoid interfering with this clamp

voltage. The minimum source current increases as VCOMP approaches VCLAMP.

Note 6. Gain is defined by A =

∆

COMP

, 0 ≤ VCS ≤ 0.4V.

Note 7. Parameter measured at trip point of latch with FB at 0V.

Note 8. The internal current sink on the CS pin is designed to discharge an external filter capacitor. It is not intended to be a DC sink path.

Note 9. For MIC3808, set VDD above the start threshold before setting at 10V.

Note 10. Does not include current in the external oscillator network.

Note 11. Maximum operating voltage is equal to the VDD [zener] shunt voltage. When operating at or near the shunt voltage, care must be taken to limit

the VDD pin current to less than the 20mA VDD maximum supply current rating.

Note 12. Start threshold and Zener Shunt threshold track one another.

MIC3808/3809 Micrel

MIC3808/3809 6 April 2005

Typical Characteristics

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 5 10 15

CURRENT (mA)

(V)

MIC3808 VDD

vs. IDD

0 2 4 6 8 10121416

IDD (mA)

VDD (V)

MIC3809 VDD

vs. IDD

-0.4

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

02468101214

OSCILLATOR (%)

VDD (V)

MIC3809 Oscillator Variation

vs. VDD

FREQUENCY (kHz)

RESISTANCE (kΩ)

Frequency

vs. RC Values

C = 330pF

C = 100pF

C = 220pF

C = 1000pF

C = 680pF

C = 470pF

VDD = 10V

C = 270pF

10k

100k

50k 100k 150k 200k

100

125

150

175

200

100

200

300

400

500

600

700

800

900

1000

DEAD TIME (ns)

CAPACITANCE (pF)

RC Pin Capacitance

vs. Deadtime

100

120

140

160

180

200

DEADTIME (ns)

RESISTANCE (kΩ)

RC Pin Resistance

vs. Deadtime

1.970

1.975

1.980

1.985

1.990

1.995

2.000

2.005

2.010

2.015

8 9 10 11 12 13 14 15 16

REFERENCE

(V)

MIC3808 VREFERENCE

vs. VDD

0.489

0.491

0.493

0.495

0.497

0.499

0.501

8 9 10 11 12 13 14 15 16

VTHRESHOLD (V)

VDD (V)

MIC3808 Current Limit

Threshold vs. VDD

0.493

0.495

0.497

0.499

0.501

0.503

0.505

4 6 8 10 12 14 16

VTHRESHOLD (V)

VDD (V)

MIC3809 Current Limit

Threshold vs. VDD

-3

-2

-1

-40 -20 0 20 40 60 80 100120140

FREQUENCY (%)

TEMPERATURE (°C)

MIC3809 Oscillator

Frequency Variation

vs. Temperature

VDD = 5V

VDD = 10V

1.85

1.87

1.89

1.91

1.93

1.95

1.97

1.99

2.01

2.03

2.05

4 6 8 1012141618

VREFERENCE (V)

VDD (V)

MIC3809 VREFERENCE

vs. VDD

1.97

1.98

1.99

2.00

2.01

2.02

-40 -20 0 20 40 60 80 100120140

VREFERENCE (V)

TEMPERATURE (°C)

Error Amplifier Reference

Volta

e vs. Temperature

April 2005 7 MIC3808/3809

MIC3808/3809 Micrel

100

120

140

160

180

PHASE (°)

FREQUENCY (Hz)

Error Ampifier

Phase

Magnitude

-40

-20

100

120

MAGNITUDE (dB)

110 100 10k 100K 1M 10M

-3

-2

-1

-40 -20 0 20 40 60 80 100120140

IDD (mA)

TEMPERATURE (°C)

VDD Supply Current

vs. Temperature

VDD = 10V

MIC3808/3809 Micrel

MIC3808/3809 8 April 2005

Functional Description

The MIC3808/9 is a high-speed power supply controller with

push-pull output drive capability. MIC3808 has a higher VDD

turn-on threshold and more hysteresis between VDD turn-on

and turn-off than the MIC3809. The outputs of the controller

operate in a push-pull fashion with a guaranteed dead time

between them. A block diagram of the MIC3808/9 controller

is shown in Figure 1.

VDD and Turn-on Sequence

The oscillator and output gate drive signals are disabled

when VDD is lower than the turn on threshold. Circuitry in the

output drivers eliminates glitching or random pulsing during

the start-up sequence. The oscillator is enabled when VDD is

applied and reaches the turn-on threshold. The VDD com-

parator also turns off the internal soft-start discharge FET,

slowly bringing up the COMP pin voltage.

The VDD pin is internally clamped. As VDD approaches this

clamp voltage, the VDD current will increase over the normal

current draw of the IC. Exceeding the VDD zener shunt

voltage may cause excessive power dissipation in the

MIC3808/9.

Soft-Start

The soft start feature helps reduce surge currents at the

power supply input source. An internal current source and

capacitor ramp up from 0V to near Vdd at a typical rate of

1V/ms. The softstart feature limits the output voltage of the

Functional Diagram

Oscillator

—1V

Peak Current

Comparator

Overcurrent

Comparator

PWM

Comparator

PWM

Latch

Slope = 1V/ms

Soft Start

2.2V

2.0V

0.5V

0.8V

14V

OUTA

OUTB

5GND

0.5V

1.2R

0.75V

3.65V

Error

Amplifier

COMP

Voltage

Reference

Figure 1. MIC3808 Block Diagram

error amplifier at the COMP pin. As the softstart voltage rises,

it allows the COMP pin voltage to rise, which in turn allows the

duty cycle of the output drivers to increase. The internal

softstart voltage is discharged and remains discharged dur-

ing the following conditions:

1. The VDD voltage drops below the turn-off

threshold

2. The voltage on the CS pin exceeds the

overcurrent comparator threshold

Once the internal softstart discharge FET is turned on, it

cannot be turned off until the internal softstart voltage drops

down below 0.5V. This insures a clean restart.

Oscillator

The oscillator operates at twice the switching frequency of

either OUTA or OUTB. The oscillator generates a sawtooth

waveform on the RC pin. The rising edge of the waveform is

controlled by the external resistor/capacitor combination.

The fall time is set by the on-resistance of the discharge FET

(see Figure 2). The fall time sets the delay (dead time)

between the turn-off of one output driver and the turn-on of the

other driver. A toggle flip-flop insures that drive signals to

OUTA and OUTB are alternated and therefore insures a

maximum duty cycle of less than 50% for each output driver.

Graphs of component values vs. oscillator frequency and

dead time are shown in the typical characteristic section of

this specification.

April 2005 9 MIC3808/3809

MIC3808/3809 Micrel

0.2V

VDD

OSCILLATOR

OUTPUT

Figure 2. Oscillator

The voltage source to the resistor/capacitor timing compo-

nents is VDD. The internal turn-off comparator threshold in the

oscillator circuit is VDD/2. This allows the oscillator to track

changes in VDD and minimize frequency variations in the

oscillator. The oscillator frequency can be roughly approxi-

mated using the following formula:

F_oscillator = 1.41/R*C

Where: frequency is in Hz

Resistance is in Ohms

Capacitance is in Farads.

Graphs of oscillator frequency and dead time vs component

values are shown in the Typical Characteristic section of this

specification. The recommended range of timing resistors

and capacitors is 10kΩ to 200kΩ and 100pF to 1000pF. To

minimize oscillator noise and insure a stable waveform the

following layout rules should be followed:

1. The higher impedance of capacitor values less

than 100pF may causes the oscillator circuit to

become more susceptible to noise. Parasitic pin

and etch trace capacitances become a larger

part of the total RC capacitance and may

influence the desired switching frequency.

2. The circuit board etch between the timing

resistor, capacitor, RC pin and ground must be

kept as short as possible to minimize noise

pickup and insure a stable oscillator waveform.

3. The ground lead of the capacitor must be routed

close to the ground lead of the MIC3808/9.

Current Sensing and Overcurrent Protection

The CS pin features are:

1. Peak current limit

2. Overcurrent limit

3. Internal current sense discharge

4. Front edge blanking

In current mode control, a PWM comparator uses the inductor

current signal and the error amplifier signal to determine the

operating duty cycle. In the MIC3808/9 the signal at the CS

pin is level shifted up before it reaches the PWM comparator

as shown in Figure 1. This allows operation of the error

amplifier and PWM comparator in a linear region.

There are two current limit thresholds in the MIC3808/9; peak

current limit and overcurrent limit. The normal operating

voltage at the CS pin is designed less than these thresholds.

A pulse-by-pulse current limit occurs when the inductor

current signal at the CS pin exceeds the peak current limit

threshold. The on-time is terminated for the remainder of the

switching cycle, regardless of whether OUTA or OUTB is

active.

If the signal at the CS pin goes past the peak threshold and

exceeds the overcurrent limit threshold, the overcurrent limit

comparator forces the soft start node to discharge and

initiates a soft start reset.

An internal FET discharges the CS pin at the end of the

oscillator charge time. The FET turns on when the voltage on

the RC pin reaches the upper threshold (VDD/2) and remains

on for the duration of the RC pin discharge time and for

typically 100ns after the start of the next on-time period. The

100ns period at the beginning of the on-time implements a

front edge blanking feature that prevents false triggering of

the PWM comparator due to noise spikes on the leading edge

of the current turn-on signal. The front edge blanking also

sets the minimum on-time for OUTA and OUTB. The timing

diagram for the CS pin is shown in Figure 3.

RC Pin

Max ON time

dead time

Oscillator

Reset

CS Pin

OUTA

OUTB

dead time

Front edge blanking

Minimum ON time

Figure 3. Timing Diagram

Error Amplifier

The error amplifier is part of the voltage control loop of the

power supply. The FB pin is the inverting input to the error

amplifier. The non-inverting input is internally connected to a

reference voltage. The output of the error amplifier, COMP,

is connected to the PWM comparator. A voltage divider

between the error amplifier output (COMP pin) and the PWM

comparator allows the error amplifier to operate in a linear

region for better transient response. The output of the error

amplifier (COMP pin) is limited to typically 3.65V to prevent

the COMP pin from rising up too high during startup or during

a transient condition. This feature improves the transient

response of the power supply.

MIC3808/3809 Micrel

MIC3808/3809 10 April 2005

Output Drivers

OUTA and OUTB are alternating output stages, which switch

at half the oscillator frequency. A toggle flip-flop in the

MIC3808/9 guarantee both outputs will not be on at the same

time. The RC discharge time is the dead time, where both

outputs are off. This provides an adjustable non-overlap time

to prevent shoot through currents and transformer saturation

in the power supply.

The output drivers are inhibited when VDD is below the

undervoltage threshold. Internal circuitry prevents the output

drivers from glitching high when VDD is first applied to the

MIC3808/9 controller.

Decoupling and PCB Layout

PCB layout is critical to achieve reliable, stable and efficient

operation. A ground plane is required to control EMI and

minimize the inductance in power, signal and return paths.

The following guidelines should be followed to insure proper

operation of the circuit:

• Low level signal and power grounds should be kept

separate and connected at only one location, preferably

the ground pin of the control IC. The ground signals for

the current sense, voltage feedback and oscillator

should be grouped together. The return signals for the

gate drives should be grouped together and a common

connection made at the ground pin of the controller. The

low level signals and their returns must be kept separate

from the high current and high voltage power section of

the power supply.

• Avoid running sensitive traces, such as the current

sense and voltage feedback signals next to or under

power components, such as the switching FETs and

transformer.

• If a current sense resistor is used, it’s ground end must

be located very close to the ground pin of the MIC3808/

9 controller. Careful PCB layout is necessary to keep

the high current levels in the current sense resistor from

running over the low level signals in the controller.

• A minimum 1µf bypass capacitor must be connected

directly between the VDD and GND pins of the MIC3808/

9. An additional 0.1uf capacitor between the VDD end

oscillator frequency setting resistor and the ground end

of the oscillator capacitor may be necessary if the

resistor is a distance away from the main 1µF bypass

capacitor

April 2005 11 MIC3808/3809

MIC3808/3809 Micrel

Package Information

45°

0°–8°

0.244 (6.20)

0.228 (5.79)

0.197 (5.0)

0.189 (4.8) SEATING

PLANE

0.026 (0.65)

MAX)

0.010 (0.25)

0.007 (0.18)

0.064 (1.63)

0.045 (1.14)

0.0098 (0.249)

0.0040 (0.102)

0.020 (0.51)

0.013 (0.33)

0.157 (3.99)

0.150 (3.81)

0.050 (1.27)

TYP

PIN 1

DIMENSIONS:

INCHES (MM)

0.050 (1.27)

0.016 (0.40)

8-Pin SOIC (M)

0.008 (0.20)

0.004 (0.10)

0.039 (0.99)

0.035 (0.89)

0.021 (0.53)

0.012 (0.03) R

0.0256 (0.65) TYP

0.012 (0.30) R

5° MAX

0° MIN

0.122 (3.10)

0.112 (2.84)

0.120 (3.05)

0.116 (2.95)

0.012 (0.3)

0.007 (0.18)

0.005 (0.13)

0.043 (1.09)

0.038 (0.97)

0.036 (0.90)

0.032 (0.81)

DIMENSIONS:

INCH (MM)

0.199 (5.05)

0.187 (4.74)

8-Pin MSOP (MM)

MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

The information furnished by Micrel in this datasheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.

Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can

reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into

the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s

use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser’s own risk and Purchaser agrees to fully indemnify

Micrel for any damages resulting from such use or sale.