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15A ? Highly ? Integrated ? SupIRBuck TM ?
IR3477 ?
Output ? Capacitor ? Selection ??
Selection ? of ? the ? output ? capacitor ? requires ? meeting ??
voltage ? overshoot ? requirements ? during ? load ? removal, ? and ?
meeting ? steady ? state ? output ? ripple ? voltage ? requirements. ??
The ? output ? capacitor ? is ? the ? most ? expensive ? converter ?
component ? and ? increases ? the ? overall ? system ? cost. ??
The ? output ? capacitor ? decoupling ? in ? the ? converter ? typically ?
includes ? the ? low ? frequency ? capacitor, ? such ? as ? Specialty ?
Polymer ? Aluminum, ? and ? mid ? frequency ? ceramic ? capacitors. ??
VESR ? is ? usually ? much ? greater ? than ? VESL. ? The ? IR3477 ?
requires ? a ? total ? ESR ? such ? that ? the ? ripple ? voltage ? at ? the ??
FB ? pin ? is ? greater ? than ? 7mV. ?
The ? second ? purpose ? of ? the ? output ? capacitor ? is ? to ? minimize ?
the ? overshoot ? of ? the ? output ? voltage ? when ? the ? load ?
decreases ? as ? shown ? in ? Figure ? 29. ? By ? using ? the ? law ? of ?
energy ? before ? and ? after ? the ? load ? removal, ? equation ? 8 ?
shows ? the ? output ? capacitance ? requirement ? for ? a ? load ??
step ? down. ??
The ? first ? purpose ? of ? output ? capacitors ? is ? to ? provide ? current ?
when ? the ? load ? demand ? exceeds ? the ? inductor ? current, ??
as ? shown ? in ? Figure ? 28. ? Equation ? 7 ? shows ? the ? charge ?
requirement ? for ? a ? certain ? load ? step. ? The ? advantage ?
provided ? by ? the ? IR3477 ? at ? a ? load ? step ? is ? the ? reduced ? delay ?
compared ? to ? a ? fixed ? frequency ? control ? method. ? If ? the ??
load ? increases ? right ? after ? the ? PWM ? signal ? goes ? low, ? the ?
longest ? delay ? will ? be ? equal ? to ? the ? minimum ? lower ? gate ??
?
?
?
?
V OUT
V DROP
V ESR
I OUT
V L
I STEP
V OS
on ‐ time ? as ? shown ? in ? the ? Electrical ? Specifications ? section. ??
The ? IR3477 ? also ? reduces ? the ? inductor ? current ? slew ? time, ?
the ? time ? it ? takes ? for ? the ? inductor ? current ? to ? reach ? equality ?
Figure ? 29: ? Typical ? Output ? Voltage ? Response ? Waveform ?
with ? the ? output ? current, ? by ? increasing ? the ? switching ?
frequency ? up ? to ? 1/( T ON ? + ? Min ? Off ? Time ). ? This ? results ? in ?
reduced ? recovery ? time. ??
?
?
C OUT
=
L ? I STEP 2
V OS 2 ? V OUT 2
(8)
Load
Current
Output
I STEP
Boot ? Capacitor ? Selection ??
The ? boot ? capacitor ? starts ? the ? cycle ? fully ? charged ? to ? a ?
?
Charge
Inductor
Slew
voltage ? of ? VB(0). ? Cg ? equals ? 0.58nF ? in ? IR3477. ? Choose ? a ?
sufficiently ? small ?Δ V ? such ? that ? VB(0) ‐Δ V ? exceeds ? the ?
maximum ? gate ? threshold ? voltage ? to ? turn ? on ? the ? upper ?
C BOOT = C g ? ? B
? 1 ? (9)
Δ t
Rate
t
MOSFET. ??
?
? V (0)
? Δ V
?
?
Figure ? 28: ? Charge ? Requirement ? during ? Load ? Step ??
?
Choose ? a ? boot ? capacitor ? value ? larger ? than ? the ? calculated ?
1 ? 1 L ? I STEP 2 ?
?
V DROP ? 2 ( V IN ? V OUT ) ?
?
Q = C ? V = 0.5 ? I STEP ? Δ t (7a)
C OUT = ? ? (7b)
C BOOT ? in ? equation ? 9. ? Equation ? 9 ? is ? based ? on ? charge ? balance ?
at ? CCM ? operation. ? Usually ? the ? boot ? capacitor ? will ? be ?
discharged ? to ? a ? much ? lower ? voltage ? when ? the ? circuit ? is ?
operating ? in ? DCM ? mode ? at ? light ? load, ? due ? to ? much ? longer ?
lower ? MOSFET ? off ? time ? and ? the ? bias ? current ? drawn ? by ? the ?
IC. ? Boot ? capacitance ? needs ? to ? be ? increased ? if ? insufficient ?
The ? output ? voltage ? drop, ? V DROP , ? initially ? depends ? on ? the ?
characteristic ? of ? the ? output ? capacitor. ? V DROP ? is ? the ? sum ? of ?
the ? equivalent ? series ? inductance ? (ESL) ? of ? the ? output ?
capacitor ? times ? the ? rate ? of ? change ? of ? the ? output ? current ?
and ? the ? ESR ? times ? the ? change ? of ? the ? output ? current. ???
turn ‐ on ? of ? the ? upper ? MOSFET ? is ? observed ? at ? light ? load, ?
typically ? larger ? than ? 0.1μF ? is ? needed. ?? The ? voltage ? rating ? of ?
this ? part ? needs ? to ? be ? larger ? than ? VB(0) ? plus ? the ? desired ?
derating ? voltage. ? It’s ? ESR ? and ? ESL ? needs ? to ? be ? low ? in ? order ?
to ? allow ? it ? to ? deliver ? the ? large ? current ? and ? di/dt’s ? which ?
drive ? MOSFETs ? most ? efficiently. ? In ? support ? of ? these ?
requirements ? a ? ceramic ? capacitor ? should ? be ? chosen. ?
15
March ? 27, ? 2013 ?? | ??? V2.5 ?? | ?? PD97604
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