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| 型号： | MIC2168YMM |
| 厂商： | Micrel Inc |
| 文件页数： | 8/14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 |
I G[HIGH -SIDE](AVG) = Q G S F
? V IN ?
T T =
V OUT × (V IN ( MAX ) ? V OUT )
MIC2168
V DD regulator operates in dropout mode, and it is necessary
that the power MOSFETs used are low threshold 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
at V GS = 4.5V must be used.
It is important to note the on-resistance of a MOSFET increases
with increasing temperature. A 75°C rise in junction tempera-
ture will increase the channel resistance of the MOSFET by
50% to 75% of the resistance speci?ed 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 speci?ed operating conditions
(V DS and V GS ). The gate charge is supplied by the MIC2168
gate drive circuit. At 1MHz switching frequency and above, the
gate charge can be a signi?cant source of power dissipation
in the MIC2168. At low output load, this power dissipation is
noticeable as a reduction in ef?ciency. The average current
required to drive the high-side MOSFET is:
×
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 usu-
ally 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
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2168 due to gate
drive is:
P GATEDRIVE = V IN ( I G[HIGH -SIDE](AVG) + I G[LOW -SIDE](AVG) )
A convenient ?gure of merit for switching MOSFETs is the on
resistance times the total gate charge R DS(ON) × Q G . Lower
numbers translate into higher ef?ciency. Low gate-charge
logic-level MOSFETs are a good choice for use with the
MIC2168.
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 )
Micrel, Inc.
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 O ?
D = DUTY CYCLE ? ?
Making the assumption the turn-on and turn-off transition times
are equal; the transition times can be approximated by:
C ISS × V GS + C OSS × V IN
I G
where:
C ISS and C OSS are measured at V DS = 0
I G = gate-drive current (1A for the MIC2168)
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)
V D = freewheeling diode drop, typically 0.5V
f S it the switching frequency, nominally 1MHz
The low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
Inductor Selection
Values for inductance, peak, and RMS currents are required
to select the output inductor. The input and output voltages
and the inductance value determine the peak-to-peak induc-
tor ripple current. Generally, higher inductance values are
used with higher input voltages. Larger peak-to-peak ripple
currents will increase the power dissipation in the inductor
and MOSFETs. Larger output ripple currents will also require
more output capacitance to smooth out the larger ripple cur-
rent. Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more expensive
inductor. A good compromise between size, loss and cost is
to set the inductor ripple current to be equal to 20% of the
maximum output current. The inductance value is calculated
by the equation below.
L =
V IN ( MAX ) × F S × 0.2 × I OUT ( MAX )
where:
f S = switching frequency, 1MHz
0.2 = ratio of AC ripple current to DC output current
V IN (max) = maximum input voltage
The peak-to-peak inductor current (AC ripple current) is:
M9999-040805
8
April 2005
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| MIC2168YMM TR | 功能描述:直流/直流开关调节器 Replaced by MIC2168AYMM TR (Lead Free) RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
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| MIC2169_09 | 制造商:MICREL 制造商全称:Micrel Semiconductor 功能描述:500kHz PWM Synchronous Buck Control IC |
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