参数资料
| 型号： | MIC2199BML TR |
| 厂商： | Micrel Inc |
| 文件页数： | 10/14页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 12-MLF |
| 标准包装： | 5,000 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 330kHz |
| 占空比： | 85% |
| 电源电压： | 4.5 V ~ 32 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： | 12-VFDFN 裸露焊盘，12-MLF? |
| 包装： | 带卷 (TR) |
| 其它名称： | MIC2199BMLTR MIC2199BMLTR-ND |
I
D × ? I OUT(max)2 + PP ?
I SW(high ? side)(rms) =
t T =
I PP2 ?
( 1 ? D ) ? I OUT(max)2 +
I SW(low ? side)(rms) = ?
V OUT
Micrel, Inc.
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
with V GS = 5V.
f s = 300kHz
The low-side MOSFET is turned on and off at V DS = 0 because
the freewheeling diode is conducting during this time. The
switching losses for the low-side MOSFET is usually negli -
gible. Also, the gate drive current for the low-side MOSFET
is more accurately calculated using C ISS 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 MIC2199 due to gate
drive is:
P GATEDRIVE = V IN ( I G[high-side](avg) + I G[low-side](avg) )
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 MIC2199. Power dissipation in the MIC2199 package
limits the maximum gate drive current.
Parameters that are important to MOSFET switch selection
are:
? Voltage rating
? On-resistance
? Total gate charge
The voltage rating of the MOSFETs 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.
Making the assumption the turn-on and turnoff transition times
are equal, the transition time can be approximated by:
C ISS × V GS + C OSS × V IN
I G
MIC2199
where:
C ISS and C OSS are measured at V DS = 0.
I G = gate drive current (1A for the MIC2199)
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 300kHz
The low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
RMS Current and MOSFET Power Dissipation
Calculation
Under normal operation, the high-side MOSFETs RMS cur -
rent is greatest when V IN is low (maximum duty cycle). The
low-side MOSFETs RMS current is greatest when V IN is high
(minimum duty cycle). However, the maximum stress the
MOSFETs see occurs during short circuit conditions, where
the output current is equal to I OVERCURRENT(max) . (See the
“Sense Resistor” section). The calculations below are for
normal operation. To calculate the stress under short circuit
conditions, substitute I OVERCURRENT(max) for I OUT(max) . Use
the formula below to calculate D under short circuit condi-
tions.
D SHORTCIRCUIT = 0.063 ? 1.8 × 10 ? 3 × V IN
The RMS value of the high-side switch current is:
? 2 ?
? 12 ?
?
? 12 ?
where:
D = duty cycle of the converter
D =
η × V IN
η = efficiency of the converter.
Converter efficiency depends on component parameters,
which have not yet been selected. For design purposes, an
efficiency of 90% can be used for V IN less than 10V and 85%
can be used for V IN greater than 10V. The efficiency can be
more accurately calculated once the design is complete. If the
assumed efficiency is grossly inaccurate, a second iteration
through the design procedure can be made.
For the high-side switch, the maximum DC power dissipa-
tion is:
P SWITCH 1(dc) = R DS(on) 1 × I SW 1(rms) 2
January 2010
10
M9999-011310
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