MIC2199 MICREL [Micrel Semiconductor], MIC2199 Datasheet - Page 11

• Manufacturer Part Number: MIC2199
• Manufacturer: MICREL [Micrel Semiconductor]

reduction in efficiency. The average current required to drive
the high-side MOSFET is:
where:
The low-side MOSFET is turned on and off at V
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
0 instead of gate charge.
For the low-side MOSFET:
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2199 due to gate
drive is:
A convenient figure of merit for switching MOSFETs is the on-
resistance times the total gate charge (R
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:
The voltage rating of the MOSFETs are essentially equal to
the input voltage. A safety factor of 20% should be added to
the V
due to circuit parasitics.
The power dissipated in the switching transistor is the sum of
the conduction losses during the on-time (P
the switching losses that occur during the period of time when
the MOSFETs turn on and off (P
where:
Making the assumption the turn-on and turnoff transition
times are equal, the transition time can be approximated by:
November 2004
MIC2199
• Voltage rating
• On-resistance
• Total gate charge
DS(max)
I
Q
f
R
I
I
P
P
P
P
G[high-side](avg)
s
G[high-side](avg)
G[low-side](avg)
GATEDRIVE
SW
CONDUCTION
AC
SW
G
= 300kHz
average high-side MOSFET gate current
taken from manufacturer’s data sheet
with V
= total gate charge for the high-side MOSFET
=
= on-resistance of the MOSFET switch.
=
P
P
of the MOSFETs to account for voltage spikes
AC(off)
CONDUCTION
GS
= 5V.
=
+
=
V
=
=
=
IN G[high-side](avg)
P
I
C
SW(rms)
AC(on)
Q
(
ISS
I
G
+
×
×
P
f
S
V
AC
2
GS
×
AC
R
×
SW
).
f
S
+
DS(on)
I
G[low-side](avg)
CONDUCTION
× Q
ISS
G
at V
). Lower
DS
) and
DS
)
= 0
=
11
where:
The total high-side MOSFET switching loss is:
where:
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
low-side MOSFETs RMS current is greatest when V
(minimum duty cycle). However, the maximum stress the
MOSFETs see occurs during short circuit conditions, where
the output current is equal to I
“Sense Resistor”
normal operation. To calculate the stress under short circuit
conditions, substitute I
the formula below to calculate D under short circuit condi-
tions.
The RMS value of the high-side switch current is:
where:
Converter efficiency depends on component parameters,
which have not yet been selected. For design purposes, an
efficiency of 90% can be used for V
can be used for V
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.
C
I
t
V
f
D = duty cycle of the converter
η = efficiency of the converter.
P
D
I
I
D
t
G
T
S
SW(high side)(rms)
SW(low side)(rms)
T
AC
D
ISS
SHORTCIRCUIT
= switching transition time (typically 20ns to 50ns)
=
it the switching frequency, nominally 300kHz
= gate drive current (1A for the MIC2199)
=
= freewheeling diode drop, typically 0.5V.
η
=
V
C
and C
×
OUT
(V
ISS
−
−
V
IN
IN
×
+
OSS
IN
V
V ) I
section). The calculations below are for
GS
D
greater than 10V. The efficiency can be
are measured at V
I
×
=
+
G
OVERCURRENT(max)
=
IN
=
PK
0.063 1.8 10
C
is low (maximum duty cycle). The
OSS
(
×
D
1 D I
t
−
×
T
−
×
OVERCURRENT(max)
⎛
⎜
⎝
×
V
I
)
OUT(max)
⎛
⎜
⎝
f
IN
S
IN
OUT(max)
×
less than 10V and 85%
−
DS
3
2
for I
×
+
2
= 0.
V
I
+
IN
PP
12
OUT(max)
I
PP
12
2
⎞
⎟
⎠
. (See the
2
MIC2199
IN
⎞
⎟
⎠
is high
Micrel
. Use