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SC453 Arkusz danych(PDF) 16 Page - Semtech Corporation

Numer części SC453
Szczegółowy opis  Portable, VID Programmed Single Phase Power Supply Controller
PDF  23 Pages
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Producent  SEMTECH [Semtech Corporation]
Strona internetowa  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC453 Arkusz danych(HTML) 16 Page - Semtech Corporation

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© 2006 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC453
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.
IRIPPLE
VINMAX VMAX_FL
−
()dMIN
⋅
L1 FS
⋅
L1
LMIN
⋅
:=
It0 IMAX_FL
IRIPPLE
2
+
:=
Load is stepping from high to low.
Immediately after the load steps down from I
MAX 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 V
FL / 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.
Applications Information (Cont.)
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.
VESR tNCAP
,
() ICAP t()
RESR
NCAP
⋅
:=
K.
dVCAP tNCAP
,
()
ICAP t
()t
⋅
COUT NCAP
⋅
:=
L.
VTOTAL tNCAP
,
() VESR tNCAP
,
() dVCAP tNCAP
,
()
+
:=
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.
LRDS
RCU
ESR_eq
IMAX
IMIN
Cout_eq
Rcu_rt
Rds_ON
BG
R_LOAD
t0 10n s
⋅
,
10
μ s
⋅
..
:=
IL t
()
It0
VMAX_FL t⋅
L1
−
⎛
⎜
⎝
⎞
⎠
:=
ICAP t
()
IL t
()
ILKGMAX
−
:=
IRIPPLE 6.01 A
=
It0 23.005 A
=



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