MIC38C42 Micrel Semiconductor, MIC38C42 Datasheet - Page 7

• Manufacturer Part Number: MIC38C42
• Description: BiCMOS Current-Mode PWM Controllers
• Manufacturer: Micrel Semiconductor

MIC38C42/3/4/5
Application Information
Familiarity with 384x converter designs is assumed.
The MIC38C4x has been designed to be compatible with
384xA series controllers.
MIC38C4x Advantages
Start-up Current
Start-up current has been reduced to an ultra-low 50µA
(typical) permitting higher-valued, lower-wattage, start-up
resistors (powers controller during power supply start-up).
The reduced resistor wattage reduces cost and printed circuit
space.
Operating Current
Operating current has been reduced to 4mA compared to
11mA for a typical bipolar controller. The controller runs
cooler and the V
start-up may be reduced.
Output Driver
Complementary internal P- and N-channel MOSFETs pro-
duce rail-to-rail output voltages for better performance driving
external power MOSFETs. The driver transistor’s low on-
resistance and high peak current capability can drive gate
capacitances of greater than 1000pF. The value of output
capacitance which can be driven is determined only by the
rise/fall time requirements. Within the restrictions of output
capacity and controller power dissipation, maximum switch-
ing frequency can approach 500kHz.
Design Precautions
When operating near 20V, circuit transients can easily ex-
ceed the 20V absolute maximum rating, permanently damag-
ing the controller’s CMOS construction. To reduce tran-
sients, use a 0.1µF low-ESR capacitor to next to the controller’s
August 2000
M17Z105
26V to 40V
1/4W
V
D2
IN
MBR030
DD
D3
R2
68k
hold-up capacitance required during
0.22µF
6.8k
R4
18
100k
Figure 1. 500kHz, 25W, Buck Converter
0.1µF*
C7
200pF
1
2
3
4
COMP
FB
ISNS
RT/CT GND
MIC38C42
CT1
16k
VREF
R5
VDD
OUT
MKS2
8
7
6
5
0.1µF
4.7µF
C5
0.1µF
7
C8
1N765B
supply V
Film type capacitors, such as Wima MKS2, are recom-
mended.
When designing high-frequency converters, avoid capacitive
and inductive coupling of the switching waveform into high-
impedance circuitry such as the error amplifier, oscillator, and
current sense amplifier. Avoid long printed-circuit traces and
component leads. Locate oscillator and compensation cir-
cuitry near the IC. Use high frequency decoupling capacitors
on V
directly to their source and use large area ground planes.
Buck Converter
Refer to figure 1. When at least 26V is applied to the input,
C5 is charged through R2 until the voltage V
14.5V (the undervoltage lockout value of the MIC38C42).
Output switching begins when Q1 is turned on by the gate
drive transformer T1, charging the output filter capacitor C3
through L1. D5 supplies a regulated +12V to V
circuit is running.
Current sense transformer CT1 provides current feedback to
ISNS for current-mode operation and cycle-by-cycle current
limiting. This is more efficient than a high-power sense
resistor and provides the required ground-referenced level
shift.
When Q1 turns off, current flow continues from ground
through D1 and L1 until Q1 is turned on again.
The 100V Schottky diode D1 reduces the forward voltage
drop in the main current path, resulting in higher efficiency
than could be accomplished using an ultra-fast-recovery
diode. R1 and C2 suppress parasitic oscillations from D1.
Using a high-value inductance for L1 and a low-ESR capaci-
tor for C3 permits small capacitance with minimum output
4.7Ω
0.1µF
D4
T1
IRF820
REF
Q1
*Locate near MIC38C42 supply pins
, and if necessary, on V
DD
(or V
1000pF
R1
10
1/2W
C2
D
for ‘-1’ versions) and ground connections.
31DQ10
L1 48µH
D1
1N4001
D5
3.3µF
C3
DD
0.1µF
. Return high di/dt currents
C4
6.19k
1.62k
1%
1%
12V, 2A
DD
V
MIC38C42/3/4/5
OUT
is greater than
DD
once the
Micrel