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| 型号： | MIC2169YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 7/15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 |
( V IN – V OUT )
V IN × F SWITCHING × L
t4 = ? OUT ? × 0.5 ×
MIC2169
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. From 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 ? Cap_COMP
? 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 MIC2169 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.
However, 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 MIC2169 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 MIC2169 current limiting circuit.
V IN
Micrel
where:
Inductor Ripple Current =
V OUT ×
F SWITCHING = 500kHz
200 μ A is the internal sink current to program the MIC2169
current limit.
The MOSFET R DS(ON) varies 30% to 40% with temperature;
therefore, it is recommended that a 50% margin be added
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 the
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) . To make the MIC2169 insensitive to
board layout and noise generated by the switch node. For
this a 1.4 Ω resistor and a 1000pF capacitor is recommended
between the switch node and ground. 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 MIC2169 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 5 Ω resistor for input supplies
between 2.9V to 5V.
MOSFET Gate Drive
The MIC2169 high-side drive circuit is designed to switch an
N-Channel MOSFET. The block diagram on page 6 shows a
bootstrap circuit, consisting of D1 and CBST. It supplies energy
to the high-side drive circuit. Capacitor CBST is charged while
C2
C IN
0.1 μ F
HSD
Q1
MOSFET N
the low-side MOSFET is on and the voltage on the VSW pin
is approximately 0V. When the high-side MOSFET driver is
RCS
CS
2 Ω
LSD
Q2
MOSFET N
L1 Inductor
1.4 Ω
1000pF
V OUT
C1
C OUT
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 D1 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,
R CS =
200 μ A
I L = I LOAD +
200μA
Figure 1. The MIC2169 Current Limiting Circuit
The current limiting resistor R CS is calculated by the follow-
ing equation:
R DS(ON) Q1 × I L
Equation (1)
1
2 ( Inductor Ripple Current )
CBST is recharged through D1. 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 D1.
An approximate 20ns delay between the high- and low-side
driver transition is used to prevent current from simultane-
ously ? owing unimpeded through both MOSFETs.
MOSFET Selection
The MIC2169 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
V DD regulator operates in dropout mode, and it is necessary
that the power MOSFETs used are sub-logic level and are in
full conduction mode for V GS of 2.5V. For applications when
V IN > 5V; logic-level MOSFETs, whose operation is speci ? ed
March 2009
7
M9999-032409
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