Datasheet

ADP1850 Data Sheet
Rev. A | Page 20 of 32
The high-side MOSFET transition loss is approximated by the
equation
2
)(
SW
FR
LOAD
IN
T
fttIV
P
×+××
≅
where:
P
T
is the high-side MOSFET switching loss power.
t
R
is the rise time in charging the high-side MOSFET.
t
F
is the fall time in discharging the high-side MOSFET.
t
R
and t
F
can be estimated by
RISEDRIVER
GSW
R
I
Q
t
_
≅
FALLDRIVER
GSW
F
I
Q
t
_
≅
where:
Q
GSW
is the gate charge of the MOSFET during switching and is
given in the MOSFET data sheet.
I
DRIVER_RISE
and I
DRIVER_FALL
are the driver current put out by the
ADP1850 internal gate drivers.
If Q
GSW
is not given in the data sheet, it can be approximated by
2
GS
GDGSW
Q
QQ +≅
where:
Q
GD
and Q
GS
are the gate-to-drain and gate-to-source charges
given in the MOSFET data sheet.
I
DRIVER_RISE
and I
DRIVER_FALL
can be estimated by
GATE
SOURCEON
SP
DD
RISEDRIVER
RR
VV
I
+
−
≅
_
_
GATE
SINKON
SP
FALLDRIVER
RR
V
I
+
≅
_
_
where:
V
DD
is the input supply voltage to the driver and is between 2.75 V
and 5 V, depending on the input voltage.
V
SP
is the switching point where the MOSFET fully conducts;
this voltage can be estimated by inspecting the gate charge
graph given in the MOSFET data sheet.
R
ON_SOURCE
is the on resistance of the ADP1850 internal driver,
given in Table 1 when charging the MOSFET.
R
ON_SINK
is the on resistance of the ADP1850 internal driver,
given in Table 1 when discharging the MOSFET.
R
GATE
is the on gate resistance of MOSFET given in the
MOSFET data sheet. If an external gate resistor is added, add
this external resistance to R
GATE
.
The total power dissipation of the high-side MOSFET is the
sum of conduction and transition losses:
T
CHS
PPP
+
≅
The synchronous rectifier, or low-side MOSFET, carries the
inductor current when the high-side MOSFET is off. The low-
side MOSFET transition loss is small and can be neglected in
the calculation. For high input voltage and low output voltage,
the low-side MOSFET carries the current most of the time.
Therefore, to achieve high efficiency, it is critical to optimize
the low-side MOSFET for low on resistance. In cases where the
power loss exceeds the MOSFET rating or lower resistance is
required than is available in a single MOSFET, connect multiple
low-side MOSFETs in parallel. The equation for low-side
MOSFET conduction power loss is
⎥
⎦
⎤
⎢
⎣
⎡
−×≅
IN
OUT
DSONLOADCLS
V
V
RIP 1)(
2
There is also additional power loss during the time, known as
dead time, between the turn-off of the high-side switch and the
turn-on of the low-side switch, when the body diode of the low-
side MOSFET conducts the output current. The power loss in
the body diode is given by
OSW
DF
BODYDIODE
IftVP ×
×
×
=
where:
V
F
is the forward voltage drop of the body diode, typically 0.7 V.
t
D
is the dead time in the ADP1850, typically 30 ns when driving
some medium-size MOSFETs with input capacitance, C
iss
, of
approximately 3 nF. The dead time is not fixed. Its effective
value varies with gate drive resistance and C
iss
, so P
BODYDIODE
increases in high load current designs and low voltage designs.
Then the power loss in the low-side MOSFET is
BODYDIODECLSLS
PPP
+
=
Note that MOSFET, R
DSON
, increases with increasing tempera-
ture with a typical temperature coefficient of 0.4%/
o
C. The
MOSFET junction temperature (T
J
) rise over the ambient
temperature is
T
J
= T
A
+ θ
JA
× P
D
where:
θ
JA
is the thermal resistance of the MOSFET package.
T
A
is the ambient temperature.
P
D
is the total power dissipated in the MOSFET.