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参数资料
| 型号： | MIC38C45YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 7/11页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST FLYBK ISO 8MSOP |
| 标准包装： | 100 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 500kHz |
| 占空比： | 50% |
| 电源电压： | 7.6 V ~ 20 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 是 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 8-TSSOP，8-MSOP（0.118"，3.00mm 宽） |
| 包装： | 管件 |
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 (typi-
cal) 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 switching
frequency can approach 500kHz.
Design Precautions
When operating near 20V, circuit transients can easily exceed
the 20V absolute maximum rating, permanently damaging the
controller ’s CMOS construction. To reduce transients, 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 recommended.
Micrel, Inc.
When designing high-frequency converters, avoid capacitive
and inductive coupling of the switching waveform into high-
impedance circuitry such as the error ampli?er, oscillator, and
current sense ampli?er. Avoid long printed-circuit traces and
component leads. Locate oscillator and compensation circuitry
near the IC. Use high frequency decoupling capacitors on
V REF , and if necessary, on V DD . Return high d i/ d t currents
directly to their source and use large area ground planes.
Buck Converter
Refer to ?gure 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 ?lter 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 ef?cient than a high-power sense resistor
and provides the required ground-referenced level shift.
When Q1 turns off, current ?ow 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 ef?ciency 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 ef?ciency
by reducing the ?ux swing in L1.
Magnetic components are carefully chosen for minimal loss
at 500kHz. CT1 and T1 are wound on Magnetics, Inc. P-
type material toroids. L1 is wound on a Siemens N49 EFD
core.
V IN
26V TO 40V
CT1
Q1
IRF820
V OUT
12V, 2A
100K
D2
M17Z105
1/4W
R2
68K
6.8K
0.1μF* MKS2
MIC38C42
C5
4.7μF
D4
1N765B
4.7 ?
0.1μF
R1
10
1/2W
C2
1000PF
L1 48μH
31DQ10
D1
C3
3.3μF
C4
0.1μF
D3
MBR030
0.22μF
R4
18
1
2
3
4
COM P V R E F
FB VDD
ISNS OU T
RT/CT GND
8
7
6
5
C8
0.1μF
T1
D5
1N4001
6.19K
1%
1.62K
1%
C7
200PF
R5
16K
0.1μF
*LOCATE NEAR MIC38C42 SUPPLY PINS
Figure 1. 500kHz, 25W, Buck Converter
September 2007
7
M9999-091107
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