9
2007 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC1189
Component Selection
SS
SS
S
WITWIT
WITWIT
WIT
CHING SECTIONCHING SECTION
CHING SECTIONCHING SECTION
CHING SECTION
OUTPUT CAPOUTPUT CAP
OUTPUT CAPOUTPUT CAP
OUTPUT CAP
AA
AA
A
CITCIT
CITCIT
CIT
ORSORS
ORSORS
ORS - Selection begins with the most
critical component. Because of fast transient load current
requirements in modern microprocessor core supplies, the
output capacitors must supply all transient load current
requirements until the current in the output inductor ramps
up to the new level. Output capacitor ESR is therefore one
of the most important criteria. The maximum ESR can be
simply calculated from:
step current Transient I
excursion voltage transient MaximumV
Where
I
V
R
t
t
t
t
ESR
=
=
≤
For example, to meet a 100mV transient limit with a 10A
load step, the output capacitor ESR must be less than
10mΩ. To meet this kind of ESR level, there are three
available capacitor technologies.
ygolonhceT
.paChcaE
.ytQ
.dqR
latoT
C
(µ )F
RSE
m( Ω)
C
(µ )F
RSE
m( Ω)
mulatnaTRSEwoL033066000201
NOC-SO0335230993.8
munimulARSEwoL005144500573.8
The choice of which to use is simply a cost/performance
issue, with Low ESR Aluminum being the cheapest, but
taking up the most space.
INDUCTORINDUCTOR
INDUCTORINDUCTOR
INDUCTOR - Having decided on a suitable type and value
of output capacitor, the maximum allowable value of in-
ductor can be calculated. Too large an inductor will pro-
duce a slow current ramp rate and will cause the output
capacitor to supply more of the transient load current
for longer - leading to an output voltage sag below the
ESR excursion calculated above.
The maximum inductor value may be calculated from:
()
OINOA
A
t
ESR
VV or V of lesser the is V where
V
I
CR
L
−
⋅≤
The calculated maximum inductor value assumes 100%
and 0% duty cycle capability, so some allowance must be
made. Choosing an inductor value of 50 to 75% of the
calculated maximum will guarantee that the inductor cur-
rent will ramp fast enough to reduce the voltage dropped
across the ESR at a faster rate than the capacitor sags,
hence ensuring a good recovery from transient with no
additional excursions.
We must also be concerned with ripple current in the out-
put inductor and a general rule of thumb has been to
allow 10% of maximum output current as ripple current.
Note that most of the output voltage ripple is produced by
the inductor ripple current flowing in the output capacitor
ESR. Ripple current can be calculated from:
OSC
IN
L
fL4
V
I
RIPPLE
⋅⋅
=
Ripple current allowance will define the minimum permit-
ted inductor value.
POPO
POPO
PO
WER FETSWER FETS
WER FETSWER FETS
WER FETS - The FETs are chosen based on several
criteria, with probably the most important being power
dissipation and power handling capability.
TT
TT
T
OP FETOP FET
OP FETOP FET
OP FET - The power dissipation in the top FET is a combi-
nation of conduction losses, switching losses and bottom
FET body diode recovery losses.
a) Conduction losses are simply calculated as:
IN
O
)on(DS
2
OCOND
V
V
cycle duty =
where
RIP
≈δ
δ⋅⋅=
b) Switching losses can be estimated by assuming a switch-
ing time, if we assume 100ns then:
2
INOSW
10VIP
−
⋅⋅=
or more generally,
4
f)tt(VI
P
OSCfrINO
SW
⋅+⋅⋅
=
c) Body diode recovery losses are more difficult to esti-
mate, but to a first approximation, it is reasonable to as-
sume that the stored charge on the bottom FET body di-
ode will be moved through the top FET as it starts to turn
on. The resulting power dissipation in the top FET will be:
OSCINRRRR
fVQP ⋅⋅=
To a first order approximation, it is convenient to only con-
