MIC2199BML(2004) 데이터 시트보기 (PDF) - Micrel
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MIC2199BML Datasheet PDF : 15 Pages
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MIC2199
reduction in efficiency. The average current required to drive
the high-side MOSFET is:
IG[high-side](avg) = QG × fS
where:
IG[high-side](avg) =
average high-side MOSFET gate current
QG = total gate charge for the high-side MOSFET
taken from manufacturer’s data sheet
with VGS = 5V.
fs = 300kHz
The low-side MOSFET is turned on and off at VDS = 0
because the freewheeling diode is conducting during this
time. The switching losses 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 VDS =
0 instead of gate charge.
For the low-side MOSFET:
IG[low-side](avg) = CISS × VGS × fS
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2199 due to gate
drive is:
( ) PGATEDRIVE = VIN IG[high-side](avg) + IG[low-side](avg)
A convenient figure of merit for switching MOSFETs is the on-
resistance times the total gate charge (RDS(on) × QG). 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 VDS(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 (PCONDUCTION) and
the switching losses that occur during the period of time when
the MOSFETs turn on and off (PAC).
PSW = PCONDUCTION + PAC
where:
PCONDUCTION = ISW(rms)2 × RSW
PAC = PAC(off) + PAC(on)
RSW = 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:
Micrel
tT
=
CISS
× VGS + COSS
IG
× VIN
where:
CISS and COSS are measured at VDS = 0.
IG = gate drive current (1A for the MIC2199)
The total high-side MOSFET switching loss is:
PAC = (VIN +VD ) × IPK × tT × fS
where:
tT = switching transition time (typically 20ns to 50ns)
VD = freewheeling diode drop, typically 0.5V.
fS 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 VIN is low (maximum duty cycle). The
low-side MOSFETs RMS current is greatest when VIN is high
(minimum duty cycle). However, the maximum stress the
MOSFETs see occurs during short circuit conditions, where
the output current is equal to IOVERCURRENT(max). (See the
“Sense Resistor” section). The calculations below are for
normal operation. To calculate the stress under short circuit
conditions, substitute IOVERCURRENT(max) for IOUT(max). Use
the formula below to calculate D under short circuit condi-
tions.
DSHORTCIRCUIT = 0.063 − 1.8 × 10−3 × VIN
The RMS value of the high-side switch current is:
ISW(high−side)(rms) =
D
×
⎛⎝⎜IOUT(max)2
+
IPP2
12
⎞
⎠⎟
ISW(low−side)(rms) =
(1−
D)
⎛⎜IOUT(max)2
⎝
+
IPP2
12
⎞
⎟
⎠
where:
D = duty cycle of the converter
D = VOUT
η × VIN
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 VIN less than 10V and 85%
can be used for VIN 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.
November 2004
11
MIC2199
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