Datasheet

ADP1828
Rev. C | Page 20 of 36
During a load step transient on the output, the output capacitor
supplies the load until the control loop has a chance to ramp the
inductor current. This initial output voltage deviation, due to a
change in load, is dependent on the output capacitor charac-
teristics. Again, usually the capacitor ESR dominates this
response, and the V
OUT
in Equation 6 can be used with the
load step current value for I
L
.
SELECTING THE MOSFETS
The choice of MOSFET directly affects the dc-to-dc converter
performance. The MOSFET must have low on resistance to
reduce I
2
R losses and low gate charge to reduce transition losses.
In addition, the MOSFET must have low thermal resistance to
ensure that the power dissipated in the MOSFET does not result
in excessive MOSFET die temperature.
The high-side MOSFET carries the load current during on-time
and usually carries most of the transition losses of the converter.
Typically, the lower the MOSFET’s on resistance, the higher the
gate charge and vice versa. Therefore, it is important to choose a
high-side MOSFET that balances the two losses. The conduction
loss of the high-side MOSFET is determined by the equation
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
≅
IN
OUT
DSONLOADC
V
V
RIP
2
)( (7)
where:
P
C
is the conduction power loss.
R
DSON
is the MOSFET on resistance.
The gate charging loss is approximated by the equation
SWG
PV
G
fQVP ≅
(8)
where:
P
G
is the gate charging loss power.
V
PV
is the gate driver supply voltage.
Q
G
is the MOSFET total gate charge.
f
SW
is the converter switching frequency.
The high-side MOSFET transition loss is approximated by the
equation
2
)(
SW
FR
LOAD
IN
T
fttIV
P
+
=
(9)
where:
P
T
is the high-side MOSFET switching loss power.
t
R
is the MOSFET rise time.
t
F
is the MOSFET fall time.
The total power dissipation of the high-side MOSFET is the
sum of all the previous losses, or
(10)
where P
HS
is the total high-side MOSFET power loss.
The conduction losses may need an adjustment to account
for the MOSFET R
DSON
variation with temperature. Note that
MOSFET R
DSON
increases with increasing temperature. The
MOSFET data sheet should list the thermal resistance of the
package, θ
JA
, along with a normalized curve of the temperature
coefficient of the R
DSON
. For the power dissipation estimated in
Equation 10, calculate the MOSFET junction temperature rise
over the ambient temperature of interest:
T
J
= T
A
+ θ
JA
P
D
(11)
Then, calculate the new R
DSON
from the temperature coefficient
curve and the R
DSON
specification at 25°C. An alternate method
to calculate the MOSFET R
DSON
at a second temperature, T
J
, is
R
DSON
@ T
J
= R
DSON
@ 25°C (1 + T
C
(T
J
− 25°C)) (12)
where T
C
is the temperature coefficient of the MOSFET’s R
DSON
,
and its typical value is 0.004/°C.
Then the conduction losses can be recalculated and the proce-
dure iterated until the junction temperature calculations are
relatively consistent.
The synchronous rectifier, or low-side MOSFET, carries the
inductor current when the high-side MOSFET is off. The low-
sition 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
he equation for low-side MOSFET
power loss is
T
GCHS
PPPP ++≅
side MOSFET tran
low-side MOSFETs in parallel. T
⎥
⎦
⎤
⎢
⎣
⎡
−≅
IN
OUT
DSONLOADLS
V
V
RIP 1)(
2
(13)
where:
P
LS
is the total low-side MOSFET power loss.
R
DSON
is the total on resistance of the low-side MOSFET(s).
Check the gate charge losses of the synchronous rectifier using
Equation 8 to be sure it is reasonable. If multiple low-side
MOSFETs are used in parallel, then use the parallel combina-
tion of the on resistances for determining RDSON to solve this
equation.