Datasheet

ADP1821
Rev. C | Page 13 of 24
In the case of output capacitors where the impedance of the
ESR and ESL are small at the switching frequency, for instance,
where the output capacitor is a bank of parallel MLCC capaci-
tors, the capacitive impedance dominates and the ripple
equation reduces to
SW
OUT
L
OUT
fC
I
V
8
Δ
≅Δ
(7)
Make sure that the ripple current rating of the output capacitors
is greater than the maximum inductor ripple current.
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 character-
istics. 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 carries all the transition losses of the converter. Typically,
the lower the MOSFET 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
(8)
where:
P
C
is the conduction power loss.
R
DSON
is the MOSFET on resistance.
The gate charging loss is approximated by the equation
SWGPVCCG
fQVP ≅
(9)
where:
P
G
is the gate charging loss power.
V
PVCC
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
+
=
(10)
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
T
GC
D
P
P
P
P
+
+
≅
(11)
where P
D
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. A 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 above, calculate
the MOSFET junction temperature rise over the ambient
temperature of interest.
T
J
= T
A
+ θ
JA
P
D
(12)
Then calculate the new R
DSON
from the temperature coefficient
curve and the R
DSON
spec at 25°C. A typical value of the temperature
coefficient (TC) of the R
DSON
is 0.004/°C, thus, 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 + TC(T
J
− 25°C)) (13)
Next, the conduction losses can be recalculated and the pro-
cedure iterated once or twice 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-
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 power loss is
()
⎥
⎦
⎤
⎢
⎣
⎡
−≅
IN
OUT
DSONLOADLS
V
V
RIP 1
2
(14)
where:
P
LS
is the low-side MOSFET on resistance.
R
DSON
is the total on resistance of the low-side MOSFET(s).
Check the gate charge losses of the synchronous rectifier
using the P
G
equation (Equation 9) to be sure it is reasonable.
If multiple low-side MOSFETs are used in parallel, then use
the parallel combination of the on resistances for determining
R
DSON
to solve this equation.