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参数资料
| 型号： | MIC2168YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 7/14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： | 1.1MHz |
| 占空比： | 90% |
| 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： | 576-1708-5 MIC2168YMM-ND |
? V OUT ? CAP_COMP
T 4 - ? ? X 0.5 X
V OUT
Q2
CS
MOSFET N
200 μ A
I L = I LOAD =
V OUT =
MIC2168
is reduced to 8.5 μ A and the COMP pin voltage rises from
0.65V to 0.95V, the bottom edge of the saw-tooth oscillator.
This is the beginning of 0% duty cycle and it increases slowly
causing the output voltage to rise slowly. The MIC2168 has
two hysteretic comparators that are enabled when V OUT is
within ±3% of steady state. When the output voltage reaches
97% of programmed output voltage, then the g m error ampli?er
is enabled along with the hysteretic comparator. This point
onwards, the voltage control loop (g m error ampli?er) is fully
in control and will regulate the output voltage.
Soft-start time can be calculated approximately by adding
the following four time frames:
t1 = Cap_COMP × 0.18V/8.5 μ A
t2 = 12 bit counter, approx 2ms
t3 = Cap_COMP × 0.3V/8.5 μ A
? V IN ? 8.5 μ A
Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 +
t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms
Current Limit
The MIC2168 uses the R DS(ON) of the top power MOSFET
to measure output current. Since it uses the drain to source
resistance of the power MOSFET, it is not very accurate. This
scheme is adequate to protect the power supply and external
components during a fault condition by cutting back the time
the top MOSFET is on if the feedback voltage is greater than
0.67V. In case of a hard short when feedback voltage is less
than 0.67V, the MIC2168 discharges the COMP capacitor to
0.65V, resets the digital counter and automatically shuts off
the top gate drive, and the g m error ampli?er and the –3%
hysteretic comparators are completely disabled and the
soft-start cycles restarts. This mode of operation is called the
“hiccup mode” and its purpose is to protect the down stream
load in case of a hard short. The circuit in Figure 1 illustrates
the MIC2168 current limiting circuit.
V IN
C2
C IN HSD Q1
MOSFET N
0
L1 Inductor
RCS
C1
LSD C OUT
200μA
Figure 1. The MIC2168 Current Limiting Circuit
Micrel, Inc.
The current limiting resistor R CS is calculated by the follow-
ing equation:
R CS = R DS(ON) Q1 × L L
Equation (1)
1
2(INDUCTOR RIPPLE CURRENT )
where:
Inductor Ripple Current =
(V IN - V OUT )
V IN × F SWITCHING × L
F SWITCHING = 1MHz
200 μ A is the internal sink current to program the MIC2168
current limit.
The MOSFET R DS(ON) varies 30% to 40% with temperature;
therefore, it is recommended to add a 50% margin to the load
current (I LOAD ) in the above equation to avoid false current
limiting due to increased MOSFET junction temperature rise.
It is also recommended to connect R CS resistor directly to
the drain of the top MOSFET Q1, and the R SW resistor to the
source of Q1 to accurately sense the MOSFETs R DS(ON) . A
0.1 μ F capacitor in parallel with R CS should be connected to
?lter some of the switching noise.
Internal V DD Supply
The MIC2168 controller internally generates V DD for self bias-
ing and to provide power to the gate drives. This V DD supply
is generated through a low-dropout regulator and generates
5V from V IN supply greater than 5V. For supply voltage less
than 5V, the V DD linear regulator is approximately 200mV
in dropout. Therefore, it is recommended to short the V DD
supply to the input supply through a 10 ? resistor for input
supplies between 2.9V to 5V.
MOSFET Gate Drive
The MIC2168 high-side drive circuit is designed to switch an
N-Channel MOSFET. The block diagram in Figure 2 shows a
bootstrap circuit, consisting of D2 and CBST, supplies energy
to the high-side drive circuit. Capacitor CBST is charged while
the low-side MOSFET is on and the voltage on the VSW pin
is approximately 0V. When the high-side MOSFET driver is
turned on, energy from CBST is used to turn the MOSFET
on. As the MOSFET turns on, the voltage on the VSW pin
increases to approximately V IN . Diode D2 is reversed biased
and CBST ?oats high while continuing to keep the high-side
MOSFET on. When the low-side switch is turned back on,
CBST is recharged through D2. The drive voltage is derived
from the internal 5V V DD bias supply. The nominal low-side
gate drive voltage is 5V and the nominal high-side gate drive
voltage is approximately 4.5V due the voltage drop across D2.
An approximate 20ns delay between the high- and low-side
driver transitions is used to prevent current from simultane-
ously ?owing unimpeded through both MOSFETs.
MOSFET Selection
The MIC2168 controller works from input voltages of 3V to
13.2V and has an internal 5V regulator to provide power to
turn the external N-Channel power MOSFETs for high- and
low-side switches. For applications where V IN < 5V, the internal
April 2005
7
M9999-040805
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