April 2005

1

MIC3838/3839

MIC3838/3839

Micrel

MIC3838/3839

Flexible Push-Pull PWM Controller

General Description

The MIC3838 and MIC3839 are a family of complementary

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

low power consumption. The MIC3838/9 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 MIC3838/9 are designed for high flexibility with minimum

pin-count. The devices are easily configurable for either

voltage-mode or current-mode control. Additionally, the

MIC3838/9 can easily implement a volt-second clamp that

automatically limits the duty cycle during input transients,

allowing designers to use the smallest possible transformers

and power components. A 3V reference output is also avail-

able that eliminates the need for an external reference.

The dual-ended push-pull architecture of the MIC3838/9

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 MIC3838 has a turn-on threshold of 12.5V whereas the

MIC3839 has a lower turn-on threshold of 4.3V. Both devices

are available in a small size MSOP-10 package 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

• Configurable for current-mode or voltage-mode control

• Easily implements volt-second clamp

• Leading edge current-sense blanking

• 3V reference output available

• 130µA typical start-up current

• 1mA typical run current

• Operation to 1MHz

• On-chip error amplifier with 4MHz gain bandwidth

product

• Internal soft start

• 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

• Industrial power supplies

• 42V automotive power supplies

• Base stations

• Networking power supplies

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

MIC3838x-x

12V 100W

VREF

RC

VDD

OUTA

COMP

GND

OUTB

RAMP

FB

ILIM

Start-Up

Circuitry

Reference

& Isolation

Feed Forward

Ramp/

Volt Sec Clamp

36V to 75V

Voltage-Mode Half-Bridge Converter CIrcuit