SC453 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC453' PDF : 23 Pages View PDF
SC453
POWER MANAGEMENT
Applications Information (Cont.)
LOAD RELEASE
The worst-case for the transient load release to happen is
when the hysteresis has just reached the maximum, (i.e.,
the high-side switch has just turned of); at this time the
inductor has reached its peak current.
( ) VINMAX − VMAX_FL ⋅dMIN L1
IRIPPLE :=
L1⋅FS
⋅
LMIN
t := 0 , 10n⋅s.. 10μ⋅s
⎛
VMAX_FL⋅t ⎞
IL(t) := ⎜ It0 −
⎝
L1 ⎠
ICAP(t) := IL(t) − ILKGMAX
IRIPPLE = 6.01 A
IRIPPLE
It0 := IMAX_FL + 2
It0 = 23.005 A
Load is stepping from high to low.
Since the output inductor is discharging at a fixed rate,
there are two terms contributing to the increase of the
voltage on the output capacitors: 1) is due to the ESR of
the output capacitor; 2) is due to the added charge con-
tributed by the inductor current.
J.
( ) RESR
VESR t, NCAP := ICAP(t)⋅ NCAP
L
Rds_ON
BG
RDS
RCU
ESR_eq
Cout_eq
K.
( ) ICAP(t)⋅t
dVCAP t, NCAP := COUT⋅NCAP
IMAX
IMIN
L.
R_LOAD
( ) ( ) ( ) VTOTAL t, NCAP := VESR t, NCAP + dVCAP t, NCAP
Rcu_rt
Immediately after the load steps down from IMAX to IMIN, the
high side FET is turned off, and the bottom FET is turned
on after the dead time. We assume for the worst-case
condition, at t = 0, the output inductor is sitting at its max-
imum; after t = 0, the inductor discharges at a rate equal
to VFL / L. (without the consideration of the secondary or-
der effect, such as, Rds_on drop, current sense resistor
and copper losses). The energy released from the output
inductor during load step-down, charges the output ca-
pacitors and is dissipated through the following means:
Rds_on, Rcs, Rcu, Rcu_rt, ESR of the output capacitors
and load.
The chart above shows the system response for the ca-
pacitors defined in Step 1 and the chart to follow shows
the details of the response for the chosen number of out-
put capactors.
© 2006 Semtech Corp.
16
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