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| 型号： | MIC2169YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 10/15页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 10-MSOP |
| 标准包装： | 100 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 550kHz |
| 占空比： | 92% |
| 电源电压： | 3 V ~ 14.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1092 (CN2011-ZH PDF) |
| 其它名称： | 576-1710-5 MIC2169YMM-ND |
MIC2169
R1
Micrel
lost in the diode is proportional to the forward voltage drop
of the diode. As the high-side MOSFET starts to turn on, the
Error
Amp
MIC2169 [adj.]
V REF
0.8V
FB
7
R2
body diode becomes a short circuit for the reverse recovery
period, dissipating additional power. The diode recovery and
the circuit inductance will cause ringing during the high-side
MOSFET turn-on. An external Schottky diode conducts at
a lower forward voltage preventing the body diode in the
MOSFET from turning on. The lower forward voltage drop
dissipates less power than the body diode. The lack of a
Figure 2. Voltage-Divider Con ? guration
Where:
V REF for the MIC2169 is typically 0.8V
The output voltage is determined by the equation:
reverse recovery mechanism in a Schottky diode causes
less ringing and less power loss. Depending upon the circuit
components and operating conditions, an external Schottky
diode will give a 1 / 2 % to 1% improvement in ef ? ciency.
Feedback Loop Compensation
V O REF × ? 1 +
= V
R2 ?
?
?
R1 ?
?
The MIC2169 controller comes with an internal transcon-
ductance error ampli ? er used for compensating the voltage
feedback loop by placing a capacitor (C1) in series with a
A typical value of R1 can be between 3k Ω and 10k Ω . If R1 is
too large, it may allow noise to be introduced into the voltage
feedback loop. If R1 is too small, in value, it will decrease the
ef ? ciency of the power supply, especially at light loads. Once
R1 is selected, R2 can be calculated using:
resistor (R1) and another capacitor C2 in parallel from the
COMP pin-to-ground. See “Functional Block Diagram.”
Power Stage
The power stage of a voltage mode controller has an induc-
tor, L1, with its winding resistance (DCR) connected to the
R2 =
V REF × R1
V O ? V REF
output capacitor, C OUT , with its electrical series resistance
(ESR) as shown in Figure 3. The transfer function G(s), for
such a system is:
External Schottky Diode
An external freewheeling diode is used to keep the inductor
current ? ow continuous while both MOSFETs are turned off.
This dead time prevents current from ? owing unimpeded
through both MOSFETs and is typically 15ns. The diode
L
DCR
ESR
C OUT
V O
conducts twice during each switching cycle. Although the
? ( 1 + ESR × s × C ) ?
2 × L × C + 1 + ESR × s × C ? ?
G(s) = ?
average current through this diode is small, the diode must
be able to handle the peak current.
I D(avg) = I OUT × 2 × 15ns × f S
The reverse voltage requirement of the diode is:
V DIODE(rrm) = V IN
The power dissipated by the Schottky diode is:
P DIODE = I D(avg) × V F
where:
V F = forward voltage at the peak diode current
The external Schottky diode, D1, is not necessary for circuit
operation since the low-side MOSFET contains a parasitic
body diode. The external diode will improve ef ? ciency and
decrease high frequency noise. If the MOSFET body diode
is used, it must be rated to handle the peak and average cur-
rent. The body diode has a relatively slow reverse recovery
Figure 3. The Output LC Filter in a Voltage Mode
Buck Converter
? DCR × s × C + s
Plotting this transfer function with the following assumed values
(L=2 μ H, DCR=0.009 Ω , C OUT =1000 μ F, ESR=0.025 Ω ) gives
lot of insight as to why one needs to compensate the loop by
adding resistor and capacitors on the COMP pin. Figures 4
and 5 show the gain curve and phase curve for the above
transfer function.
30
30
7.5
-15
time and a relatively high forward voltage drop. The power
-37.5
1.10
1 .10
1 .10
1 .10
-80 -80
100
100
3
f
4
5
6
1000000
Figure 4. The Gain Curve for G(s)
M9999-032409
10
March 2009
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