MIC3808/3809

Micrel

MIC3808/3809

2

April 2005

Pin Configuration

1

COMP

FB

CS

RC

8

VDD

OUTA

OUTB

GND

7

6

5

2

3

4

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

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

frequency stability. Only two components are required to program the

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

formula:

F

1.41

RC

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

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

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