PROGRAMMABLE SYNCHRONOUS DC/DC
CONVERTER, DUAL LOW DROPOUT
REGULATOR CONTROLLER
SC1182/3
August 25, 1998
fast enough to reduce the voltage dropped across the
ESR at a faster rate than the capacitor sags, hence en-
suring a good recovery from transient with no additional
excursions.
We must also be concerned with ripple current in the
output 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 ca-
pacitor ESR. Ripple current can be calculated from:
COMPONENT SELECTION
SWITCHING SECTION
OUTPUT CAPACITORS - 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:
V
IN
I
=
Vt
LRIPPLE
4 L fOSC
RESR
≤
It
Ripple current allowance will define the minimum per-
mitted inductor value.
Where
Vt Maximum transient voltage excursion
=
POWER FETS - The FETs are chosen based on sev-
eral criteria with probably the most important being
power dissipation and power handling capability.
TOP FET - The power dissipation in the top FET is a
combination of conduction losses, switching losses and
bottom FET body diode recovery losses.
It Transient current step
=
For example, to meet a 100mV transient limit with a
10A load step, the output capacitor ESR must be less
than 10m . To meet this kind of ESR level, there are
Ω
a) Conduction losses are simply calculated as:
three available capacitor technologies.
PCOND I2 RDS(on)
=
δ
Each Capacitor
Total
O
where
= duty cycle
Technology
C
ESR Qty.
C
ESR
(mΩ)
(µF)
Rqd. (µF)
(mΩ)
60
VO
δ
≈
Low ESR Tantalum
OS-CON
330
6
3
5
2000
990
10
8.3
8.8
VIN
330
25
44
b) Switching losses can be estimated by assuming a
switching time, if we assume 100ns then:
Low ESR Aluminum
1500
7500
2
PSW
I
VIN 10 −
=
The choice of which to use is simply a cost/perfor-
mance issue, with Low ESR Aluminum being the
cheapest, but taking up the most space.
O
or more generally,
INDUCTOR - Having decided on a suitable type and
value of output capacitor, the maximum allowable
value of inductor can be calculated. Too large an in-
ductor will produce a slow current ramp rate and will
cause the output capacitor to supply more of the tran-
sient load current for longer - leading to an output volt-
age sag below the ESR excursion calculated above.
The maximum inductor value may be calculated from:
IO VIN (tr t ) f
+
f
OSC
PSW
=
4
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
diode will be moved through the top FET as it starts to
turn on. The resulting power dissipation in the top FET
will be:
RESR
It
C
L
(
VIN
V
−
)
≤
PRR
Q
VIN fOSC
=
O
RR
The calculated maximum inductor value assumes 100% To a first order approximation, it is convenient to only
duty cycle, so some allowance must be made. Choosing consider conduction losses to determine FET suitability.
an inductor value of 50 to 75% of the calculated maxi- For a 5V in; 2.8V out at 14.2A requirement, typical FET
mum will guarantee that the inductor current will ramp
© 1998 SEMTECH CORP.
losses would be:
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