SC4524EVB View Datasheet(PDF) - Semtech Corporation
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SC4524EVB Datasheet PDF : 21 Pages
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SC4524
POWER MANAGEMENT
Applications Information
shows that the minimum on time is about 105ns at room
temperature (Figure 5). The power switch in the SC4524
is either not turned on at all or for at least TON(MIN). If the
'
required switch on time (= I ) is shorter than the minimum
on time, the regulator will either skip cycles or it will jitter.
'
=
+
+ -
=
.
The maximum operating channel frequency of the
converter
is
therefore
-'
QV
=
.+]
.
Example: Determine the maximum operating frequency
of a 24V to 1.2V switching regulator using the SC4524.
Assuming that VD = 0.45V, VCESAT = 0.25V and VIN = 26.4V
(10% high line), the duty ratio can be calculated using
(2).
' = + =
+ -
To allow for transient headroom, the minimum operating
switch on time should be at least 30% higher than the
worst-case minimum on time exhibited in Figure 5.
Designing for a switch on time of 150ns at 9 = 9 ,
,1
the
maximum
operating
frequency
is
'
QV
= .+] .
Minimum Off Time Limitation
The PWM latch in Figure 2 is reset every period by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
switching frequency. The measured minimum off time is
120ns. For a step-down converter, D increases with
9
increasing 287 ratio. If the required duty cycle is higher
9
,1
than the attainable maximum, then the output voltage
will not be able to reach its set value in continuous-
conduction mode.
Example: Determine the maximum operating frequency
of a 5V to 4V switching regulator using the SC4524.
Assuming that VD = 0.45V, VCESAT = 0.25V and VIN = 4.5V
(10% low line), the duty ratio can be calculated using (2).
Transient headroom requires that channel frequency be
lower than 410kHz.
Inductor Selection
The inductor ripple current DIL for a non-synchronous
step-down converter in continuous-conduction mode is
D ,/
=
9287
+
9'
I/
- '
=
9287
+ 9' 9,1 - 9287 - 9&(6$7
9,1 + 9' - 9&(6$7 I/
(3)
where f is the switching frequency and L is the
inductance.
In current-mode control, the slope of the modulating
(sensed switch current) ramp should be steep enough
to lessen jittery tendency but not so steep that large
flux swing decreases efficiency. Inductor ripple current
DIL between 25-40% of the peak inductor current limit is
a good compromise. Inductors so chosen are optimized
in size and DCR. Setting D,/ = = $ ,
9' = 9 and 9&(6$7 = 9 in (3),
/
=
9
287
+
9
,Q
-
9
287
-
9 + I
(4)
,1
where L is in mH and f is in MHz.
Equation (3) shows that for a given 9287 D,/ increases
as D decreases. If 9 varies over a wide range, then
,1
choose L based on the nominal input voltage. Always
verify converter operation at the input voltage extremes.
The peak current limits of both SC4524 power transistors
are internally set at 3.2A. The peak current limits are
ã 2006 Semtech Corp.
10
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