sc1109acstrt Semtech Corporation, sc1109acstrt Datasheet - Page 10

• Manufacturer Part Number: sc1109acstrt
• Description: Sc1109 Synchronous Pwm Controller With Dual Low-dropout Regulator Controllers
• Manufacturer: Semtech Corporation

COMPONENT SELECTION
SWITCHING SECTION
OUTPUT CAPACITORS - Selection begins with the most
critical component. Because of fast transient load cur-
rent requirements in modern microprocessor core sup-
plies, the output capacitors must supply all transient load
current requirements until the current in the output in-
ductor ramps up to the new level. Output capacitor ESR
is therefore one of the most important criteria. The maxi-
mum ESR can be simply calculated from:
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 three
available capacitor technologies.
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.
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.
POWER MANAGEMENT
Application Information (Cont.)
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The maximum inductor value may be calculated from:
The calculated maximum inductor value assumes 100%
duty cycle, so some allowance must be made. Choosing
an inductor value of 50 to 75% of the calculated maxi-
mum will guarantee that the inductor current will ramp
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 cur-
rent 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 capacitor ESR. Ripple current can be calculated
from:
Ripple current allowance will define the minimum permit-
ted inductor value.
POWER FETS - The FETs are chosen based on several
criteria with probably the most important being power
dissipation and power handling capability.
TOP FET - The power dissipation in the top FET is a com-
bination of conduction losses, switching losses and bot-
tom FET body diode recovery losses.
a) Conduction losses are simply calculated as:
b) Switching losses can be estimated by assuming a
switching time, if we assume 100ns then:
P
where
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SC1109