SC2620
POWER MANAGEMENT
Applications Information
that the minimum on time is about 105ns at room
temperature (Figure 5). The power switch in the SC2620
is either not turned on at all or for at least T
ON(MIN)
. If the
Example
:
Determine the maximum operating frequency of
a dual 5V to 1.5V and 5V to 4V switching regulator using
the SC2620.
Assuming that V
D
= 0.45V, V
CESAT
= 0.25V and V
IN
= 4.5V
(10% low line), the duty ratios
D
1
and
D
2
of the 1.5V and
4V converters can be calculated using (2).
D
) is shorter than the minimum
f
on time, the regulator will either skip cycles or it will jitter.
required switch on time (=
Example: Determine the maximum operating frequency
of a dual 24V to 1.2V and 24V to 3.3V switching regulator
using the SC2620.
Assuming that V
D
= 0.45V, V
CESAT
= 0.25V and V
IN
= 26.4V
(10% high line), the corresponding duty ratios, D
1
and D
2
,
of the 1.2V and 3.3V converters can be calculated using
(2).
D
1
=
1.5
+
0.45
=
0.42
4.5
+
0.45
−
0.25
4
+
0.45
=
0.95
.
4.5
+
0.45
−
0.25
D
2
=
The maximum operating channel frequency of the dual
D
1
=
1.2
+
0.45
=
0.062
26.4
+
0.45
−
0.25
3.3
+
0.45
=
0.14
26.4
+
0.45
−
0.25
1.5V and the 4V converter is therefore
1
−
D
2
=
410kHz
.
120ns
D
2
=
Transient headroom requires that channel frequency be
lower than 410kHz.
Inductor Selection
The inductor ripple current
∆I
L
for a non-synchronous step-
down converter in continuous-conduction mode is
∆
I
L
=
( V
OUT
+
V
D
)(1
−
D) ( V
OUT
+
V
D
)( V
IN
−
V
OUT
−
V
CESAT
)
=
fL
( V
IN
+
V
D
−
V
CESAT
) fL
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
V
IN
=
26.4 V
,
the maximum operating frequency of the 24V to 1.2V and
3.3V converter is
D
1
=
410kHz
.
150ns
(3)
Minimum Off Time Limitation
where f is the switching frequency and L is the inductance.
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 increasing
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
∆I
L
between 25-40% of the peak inductor current limit is a
good compromise. Inductors so chosen are optimized in
size and DCR. Setting
∆
I
L
=
0.3(2.3)
=
0.69 A
,
V
OUT
V
IN
ratio. If the required duty cycle is higher than the attainable
maximum, then the output voltage will not be able to reach
its set value in continuous-conduction mode.
V
D
=
0.45 V
and
V
CESAT
=
0.25 V
in (3),
2006 Semtech Corp.
10
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