参数资料
| 型号： | MIC2169BYMME |
| 厂商： | Micrel Inc |
| 文件页数： | 11/25页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 10-MSOP |
| 特色产品： | MIC2169B Buck Control IC |
| 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 ~ 85°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽）裸露焊盘 |
| 包装： | 管件 |
| 产品目录页面： | 1092 (CN2011-ZH PDF) |
| 其它名称： | 576-3545-5 |
D = duty cycle = ? ? O ? ?
C ISS × V GS + C OSS × V IN
t T =
Micrel, Inc.
Application Information
MOSFET Selection
The MIC2169B controller works from input voltages of
3V to 14.5V 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 specified at V GS =
4.5V must be used. For the lower (<5V) applications, the
V DD supply can be connected directly to V IN to help
increase the driver voltage to the MOSFET.
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in
junction temperature will increase the channel resistance
of the MOSFET by 50% to 75% of the resistance
specified at 25°C. This change in resistance must be
accounted for when calculating MOSFET power
dissipation and in calculating the value of current-sense
(CS) resistor. Total gate charge is the charge required to
turn the MOSFET on and off under specified operating
conditions (V DS and V GS ). The gate charge is supplied by
the MIC2169B gate-drive circuit. At 500kHz switching
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2169B.
At low output load, this power dissipation is noticeable
as a reduction in efficiency. The average current
required to drive the high-side MOSFET is:
I G [ high ? side ]( avg ) = Q G × f S
where:
I G[high-side](avg) = average high-side MOSFET gate
current.
Q G = total gate charge for the high-side MOSFET
taken from manufacturer’s data sheet for V GS = 5V.
The low-side MOSFET is turned on and off at V DS = 0
because the freewheeling diode is conducting during this
time. The switching loss for the low-side MOSFET is
usually negligible. Also, the gate-drive current for the
low-side MOSFET is more accurately calculated using
CISS at V DS = 0 instead of gate charge.
For the low-side MOSFET:
I G [ low ? side ]( avg ) = C ISS × V GS × f S
MIC2169B
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R DS(ON) × Q G .
Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2169B.
Parameters that are important to MOSFET switch
selection are:
? Voltage rating
? On-resistance
? Total gate charge
The voltage ratings for the top and bottom MOSFET are
essentially equal to the input voltage. A safety factor of
20% should be added to the V DS (max) of the MOSFETs
to account for voltage spikes due to circuit parasitics.
The power dissipated in the switching transistor is the
sum of the conduction losses during the on-time
(P CONDUCTION ) and the switching losses that occur during
the period of time when the MOSFETs turn on and off
(P AC ).
P SW = P CONDUCTION + P AC
where:
P CONDUCTION = I SW ( rms ) 2 × R SW
P AC = P AC ( off ) + P AC ( on )
R SW = on-resistance of the MOSFET switch
? V ?
? V IN ?
Making the assumption the turn-on and turn-off transition
times are equal; the transition times can be
approximated by:
I G
where:
C ISS and C OSS are measured at V DS = 0
I G = gate-drive current (1.4A for the MIC2169B)
The total high-side MOSFET switching loss is:
P AC = ( V IN + V D ) × I PK × t T × f S
where:
t T = switching transition time (typically 20ns to
50ns)
Since the current from the gate drive comes from the
input voltage, the power dissipated in the MIC2169B due
to gate drive is:
I
P GATEDRIVE = V IN × ( G [ high ? side ]( avg ) + I G [ low ? side ]( avg )
April 2010
)
11
V D = freewheeling diode drop, typically 0.5V
f S it the switching frequency, nominally 500kHz
The low-side MOSFET switching losses are negligible
and can be ignored for these calculations.
M9999-041210-B
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