MIC38C42A/43A/44A/45A Micrel, Inc.
April 2005 7 M9999-042205
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
REF
, and if necessary, on V
DD
. Return high di/dt currents
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
DD
is greater than
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
DD
once the
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
ripple. This inductance value also improves circuit efficiency
by reducing the flux swing in L1.
Application Information
Familiarity with 384x converter designs is assumed.
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
DD
hold-up capacitance required during
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
supply V
DD
(or V
D
for ‘-1’ versions) and ground connections.
Film type capacitors, such as Wima MKS2, are recom-
Figure 1. 500kHz, 25W, Buck Converter
V
OUT
12V, 2A
COMP
FB
ISNS
RT/CT GND
OUT
VDD
VREF
C2
1000pF
R1
10
1/2W
31DQ10
D1
L1 48µH
C3
3.3µF
C4
0.1µF
6.19k
1%
1.62k
1%
R4
18
C7
200pF
R5
16k
0.1µF
D3
MBR030
C8
0.1µF
T1
6.8k
0.22µF
100k
Q1
IRF820
0.1µF
C5
4.7µF
R2
68k
D2
M17Z105
1/4W
D5
1N4001
D4
1N765B
V
IN
26V to 40V
CT1
4.7Ω
MIC38C42A
1
2
3
4
8
7
6
5
0.1µF*
*Locate near MIC38C42 supply pins
MKS2
