SC453 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC453' PDF : 23 Pages View PDF
SC453
POWER MANAGEMENT
Applications Information (Cont.)
In the second condition, because the hysteretic converter
responds in < 100ns, the capacitor does not droop very
far before the inductor current starts ramping up. (This is
not true of control schemes where time constants in the
error amplifier cause delays.) Once the inductor current
starts to rise, the increasing ∆V of the capacitor is offset
by reduced ∆V from the ESR, so ∆V is constant. If the ∆V
due to the charge taken from the capacitor before the in-
ductor current reaches the load current (note the shaded
area on the graph) is less than VPOS_TRANS, then the
transient response passes.
Based on the following four factors: 1) minimum induc-
tance requirement; 2) device availability at the time we
designed the evaluation board; 3) low DCR ; 4) height and
package size consideration. Keep in mind, the choice you
make should be based upon the requirement of the con-
verter design, including minimum inductance, minimum
saturation current, efficiency, foot area, maximum allow-
able height, and of course, device availability. In the cur-
rent demo board design,
G.
L1 := 0.60⋅μH
Since the highest output voltage has the most severe re-
quirements, any other modes are satisfied by a design op-
timized for the highest output voltage.
VPOS_TRANS
C.
( ) ESRMAX := IMAX_FL − ILKGMAX
ESRMAX
=
3.333
×
−
10
3Ω
For the second condition, we need to know the inductor
value, which is a function of the highest desired switching
frequency. The maximum frequency occurs at the high-
est input voltage. As a reasonable compromise between
efficiency and component size, a maximum switching fre-
quency of 350kHz is desired.
D.
FS := 350K⋅Hz
VMAX_NL
E.
dMIN := VINMAX
F.
( ) ( ) VINMAX − VMAX_NL ⋅ ESRMAX + RCS
LMIN := dMIN⋅
⎛ ESRMAX + RCS ⎞
FS⋅VRIPPLE⋅⎜
⎝
ESRMAX
⎠
LMIN
=
5.422
×
−
10
7
H
This value of inductance is required up to maximum load.
Inductors with a “swinging choke” characteristic, where
the zero current value of inductance is much less than the
full load current inductance can be used, as long as the
above restriction is met. Then, the worst-case (low input
voltage) response time (the time for the current to reach
the new transient value) is:
( ) L1⋅ IMAX_FL − ILKGMAX
dT :=
VINMIN − VMAX_NL
H.
dT
=
1.326
×
−
10
6
s
Add ~100ns for the propagation delay from a change at
the output to the MOSFET switch turning on in reaction.
Since the shaded area is triangular, the total charge taken
out of the capacitor = (dI / dt) / 2. Q = C / dV = (dI / dt) /
2, therefore;
( ) ( ) IMAX_FL − ILKGMAX ⋅ dT + 1⋅10− 7⋅sec
I.
CMINP :=
VPOS_TRANS
CMINP
=
4.278
×
−
10
4
F
This condition applies only to the positive transient.
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15
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