MIC3808_11 Datasheet, PDF (2/11 Pages) MIC GROUP RECTIFIERS – Push-Pull PWM Controller

MIC3808/3809
Ordering Information
Part Number
Standard
MIC3808BM
MIC3809BM
MIC3808BMM
MIC3809BMM
Lead-Free
MIC3808YM
MIC3809YM
MIC3808YMM
MIC3809YMM
Turn On
Threshold
12.5V
4.3V
12.5V
4.3V
Micrel
Turn Off
Threshold
8.3V
4.1V
8.3V
4.1V
Temperature Range
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Package
8-Pin SOIC
8-Pin SOIC
8-Pin MSOP
8-Pin MSOP
Pin Configuration
Pin Description
Pin Number
1
2
3
4
COMP 1
FB 2
CS 3
RC 4
8 VDD
7 OUTA
6 OUTB
5 GND
SOIC-8 (M)
MSOP-8 (MM)
Pin Name
COMP
FB
CS
RC
Pin Function
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.
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.
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.
The oscillator programming pin. The MIC3808’s oscillator tracks V_DD 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 V_DD and RC), and a capacitor (tied to the RC
and GND). The approximate oscillator frequency is determined by the simple
formula:
F_OSCILLATOR
=
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
capacitor lead to GND as short as possible, the timing resistor lead from V_DD
as short as possible, and the leads between timing components and RC as
short as possible. Separate ground and V_DD traces to the external timing
network are encouraged.
MIC3808/3809
2
April 2005