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15A ? Highly ? Integrated ? SupIRBuck TM ?
IR3477 ?
1.25 V ? ( 21 V - 1.25 V )
21 V ? 1 μ H ? 400k Hz
?
1 ? 1 . 5 A ?
1.25 V
21 V 3 ? 1 2 A ?
R FF =
= 156 k Ω
DESIGN?EXAMPLE?
?
DESIGN ? CRITERIA ??
? Input ? Voltage, ? VIN ? = ? 6V ? to ? 21V ?
? Output ? Voltage, ? VOUT ? = ? 1.25V ?
? Switching ? Frequency, ? Fs ? = ? 400kHz ?
? Inductor ? Ripple ? Current, ?? 2 Δ I ? = ? 3A ?
? Maximum ? Output ? Current, ? IOUT ? = ? 12A ?
? Over ? Current ? Trip, ? IOC ? = ? 18A ?
? Current ? Transient ? Step ? Size ? = ? 5A ?
? Overshoot ? Allowance, ? VOS ? = ? VOUT ? + ? 50mV ?
? Undershoot ? Allowance, ? VDROP ? = ? 50mV ?
Find ? R FF : ?
1.25 V
1 V ? 20 pF ? 400k Hz
?
Pick ? a ? standard ? value ? 158 ? k Ω , ? 1% ? resistor. ?
loss ? as ? possible ? to ? increase ? the ? overall ? system ? efficiency. ??
For ? instance, ? choose ? a ? PIMB103E ‐ 1R0MS ‐ 39 ? manufactured ?
by ? CYNTEC. ? The ? inductance ? of ? this ? part ? is ? 1μH ? and ? has ?
2.7m Ω? DCR. ? Ripple ? current ? needs ? to ? be ? recalculated ? using ?
the ? chosen ? inductor. ?
2 Δ I = = 3 A
Choose ? an ? input ? capacitor: ?
2
I IN_RMS = 1 2 A ? ? 1 + ? ? ? = 2 . 9 A
?
?
A ? Panasonic ? 10μF ? (ECJ3YB1E106M) ? accommodates ? 6 ? Arms ?
of ? ripple ? current ? at ? 300kHz. ? Due ? to ? the ? chemistry ? of ?
multilayer ? ceramic ? capacitors, ? the ? capacitance ? varies ? over ?
temperature ? and ? operating ? voltage, ? both ? AC ? and ? DC. ? One ?
10μF ? capacitor ? is ? recommended. ?? In ? a ? practical ? solution, ?
one ? 1μF ? capacitor ? is ? required ? along ? with ? 10μF. ? The ?
purpose ? of ? the ? 1μF ? capacitor ? is ? to ? suppress ? the ? switching ?
noise ? and ? deliver ? high ? frequency ? current. ??
Find ? R ISET : ?
R ISET =
7.5m Ω ? 18 A
1 9 μ A
= 7.1k Ω
Choose ? an ? output ? capacitor: ?
To ? meet ? the ? undershoot ? and ? overshoot ? specification, ?
equations ? 7b ? and ? 8 ? will ? be ? used ? to ? calculate ? the ? minimum ?
V FB =
? V OUT = 0.5 V
V OUT ? ( V IN ? V OUT )
V IN ? 2 Δ I ? F s
1.25 V ? ( 21 V - 1.25 V )
R 6 =
DCR ? C 13
?
1 μ H
2 . 7 m Ω ? 100 nF
?
Pick ? a ? 7.15k Ω , ? 1% ? standard ? resistor. ??
Find ? a ? resistive ? voltage ? divider ? for ? V OUT ? = ? 1.25V: ?
R 2
R 2 + R 1
R 2 ? = ? 1.33k Ω , ? R 1 ? = ? 1.96 ? k Ω , ? both ? 1% ? standard ? resistors. ??
Choose ? the ? soft ? start ? capacitor: ?
Once ? the ? soft ? start ? time ? has ? chosen, ? such ? as ? 1000us ? to ?
reach ? to ? the ? reference ? voltage, ? a ? 22nF ? for ? CSS ? is ? used ? to ?
meet ? 1000us. ?
Choose ? an ? inductor ? to ? meet ? the ? design ? specification: ?
?
L =
?
=
21 V ? 3 A ? 400k Hz
= 1.0 μ H
?
Choose ? the ? inductor ? with ? the ? lowest ? DCR ? and ? AC ? power ?
output ? capacitance. ? As ? a ? result, ? 200 μ F ? will ? be ? needed ? for ?
5A ? load ? removal. ? To ? meet ? the ? stability ? requirement, ?
choose ? an ? output ? capacitor ? with ? ESR ? larger ? than ? 6m Ω . ?
Combine ? those ? two ? requirements, ? one ? can ? choose ? a ? set ? of ?
output ? capacitors ? from ? manufactures ? such ? as ? SP ‐ Cap ?
(Specialty ? Polymer ? Capacitor) ? from ? Panasonic ? or ? POSCAP ?
from ? Sanyo. ? A ? 220 μ F ? (EEFSL0D221R) ? from ? Panasonic ? with ?
9m Ω? ESR ? will ? meet ? both ? requirements. ??
If ? an ? all ? ceramic ? output ? capacitor ? solution ? is ? desired, ? the ?
external ? slope ? injection ? circuit ? composed ? of ? R6, ? C13, ? and ?
C14 ? is ? required ? as ? explained ? in ? the ? Stability ? Considerations ?
section. ? In ? this ? design ? example, ? we ? can ? choose ? C14 ? = ? 1nF ?
and ? C13 ? = ? 100nF. ? To ? calculate ? the ? value ? of ? R6 ? with ?
PIMB103E ‐ 1R0MS ‐ 39 ? as ? our ? inductor: ?
L
?
?
=
?
= 3 . 7 k Ω
Pick ? a ? standard ? value ? for ? R6 ? = ? 3.74k Ω . ?
16
March ? 27, ? 2013 ?? | ??? V2.5 ?? | ?? PD97604
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