MIC3838 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Flexible Push-Pull PWM Controller

MIC3838/3839
Functional Diagram
Micrel
FB
COMP
RAMP
ILIM
Overcurrent
Comparator
V_DD OK
0.75V
2.2V
V_DD Ð1V
V_REF
Voltage
Reference
0.5V
Soft Start
3.65V
2.0V
Error
Amplifier
S
Q
R
S
Q
R
V_DD
Slope = 1V/ms
Peak Current
Comparator
0.5V
Oscillator
1.2R
0.8V
PWM
Comparator
R
14V
PWM
Latch
S
Q
R
Q
T
/Q
RC
Figure 1. MIC3838 Block Diagram
V_IN = 36V to 75V
MIC3838
150k
13k
50k
4.5k
330pF
V_COMP
PWM
Comparator
0.5V
Peak Current
Comparator
330pF
0.75V
Over Current
Comparator
V_DD
OUTA
OUTB
GND
V_MAX at Ramp Input = 1.5V
V_MAX at I_LIM Input = 0.5V
Oscillator
Figure 2. Volt Second Clamp and Voltage Feed Forward Circuit
Functional Description
The MIC3838/9 is a high-speed power supply controller with
push-pull output drive capability. MIC3838 has a higher V_DD
turn-on threshold and more hysteresis between V_DD turn-on
and turn-off than the MIC3839. The outputs of the controller
operate in a push-pull fashion with a guaranteed dead time
between them. A block diagram of the MIC3838/9 controller
is shown in Figure 1.
V_DD and Turn-on Sequence
The oscillator and output gate drive signals are disabled
when V_DD 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 V_DD is
applied and reaches the turn-on threshold. The V_DD com-
parator also turns off the internal soft-start discharge FET,
slowly bringing up the COMP pin voltage.
The V_DD pin is internally clamped. As V_DD approaches this
clamp voltage, the V_DD current will increase over the normal
current draw of the IC. IDD currents greater than 20mA may
cause excessive power dissipation in the MIC3838/9.
MIC3838/3839
8
August 2003