MIC3838/3839

Micrel

MIC3838/3839

2

August 2003

Ordering Information

Turn On

Turn Off

Part Number

Threshold

Threshold

Temperature Range

Package

MIC3838BMM

12.5V

8.3V

–40

°C to +85°C

MSOP-10

MIC3839BMM

4.3V

4.1V

–40

°C to +85°C

MSOP-10

Pin Configuration

1

COMP

FB

ILIM

RAMP

10 VREF

VDD

OUTA

OUTB

9

8

7

2

3

4

RC

GND

6

5

MSOP-10 (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

comparator. The error amplifier in the MIC3838 is a true low-output imped-

ance, 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 MIC3838 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.

3

ILIM

The input to the 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

RAMP

Input to the PWM comparator. Sawtooth ramp for PWM control. Allows for

either current-mode or voltage-mode control. An internal MOSFET dis-

charges the current sense filter capacitor.

5

RC

The oscillator programming pin. Only two components are required to

program the oscillator, a resistor (tied between V

tor (tied between RC and GND). The approximate oscillator frequency is

determined by the simple formula:

F

1.41

RC

×

The recommended range of timing resistors is between 7k

Ω and 200kΩ and

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

less than 7k

Ω should be avoided. For best performance, keep the leads

between components as short as possible. Separate ground and VDD

traces to the external timing network are encouraged.

6

GND

Ground. Return path for signal and gate drive functions.