參數(shù)資料
型號(hào)： MIC2199
廠商： Micrel Semiconductor,Inc.
英文描述： 300kHz 4mm 】 4mm Synchronous Buck Converter
中文描述： 300kHz的4毫米】4mm的同步降壓轉(zhuǎn)換器
文件頁(yè)數(shù)： 11/15頁(yè)
文件大?。?/td> 222K
代理商： MIC2199
November 2004
11
MIC2199
MIC2199
Micrel
reduction in efficiency. The average current required to drive
the high-side MOSFET is:
I
Q
f
G[high-side](avg)
G
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
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 negligible. 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
C
V
f
G[low-side](avg)
ISS
GS
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
V
I
I
GATEDRIVE
IN
G[high-side](avg)
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
P
P
SW
CONDUCTION
AC
=
+
where:
P
I
R
CONDUCTION
SW(rms)
SW
2
=
×
P
P
P
AC
AC(off)
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:
t
C
V
C
V
I
T
ISS
GS
OSS
IN
G
=
×
+
×
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
(V
V ) I
+
t
f
AC
IN
PK
T
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
0.063 1.8 10
V
SHORTCIRCUIT
3
IN
=
×
The RMS value of the high-side switch current is:
I
D
I
I
12
SW(high side)(rms)
OUT(max)
2
PP
2
=
×
+
I
1 D I
(
I
12
SW(low side)(rms)
OUT(max)
2
PP
2
=
+
where:
D = duty cycle of the converter
D
V
V
OUT
×
IN
=
η
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.
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