Datasheet


SLVS612 − APRIL 2006
21
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DESIGN EXAMPLE
D Input Voltage: 10 Vdc to 14.4 Vdc
D Output voltage: 1.25 V ±1% (1.2375 ≤ V
O
≤1.2625)
D Output current: 8 A (maximum, steady state), 10 A (surge, 10ms duration, 10% duty cycle maximum)
D Output ripple: 33 mV
P-P
at 8 A
D Output load response: 0.1 V => 10% to 90% step load change, from 1 A to 7 A
D Operating temperature: −40°C to 85°C
D f
SW
=170 kHz
1. Calculate maximum and minimum duty cycles
d
MIN
+
V
O(min)
V
IN(max)
+
1.2375
14.4
+ 0.086 d
MAX
+
V
O(max)
V
IN(min)
+
1.2625
10
+ 0.126
2. Select switching frequency
The switching frequency is based on the minimum duty cycle ratio and the propagation delay of the current limit
comparator. In order to maintain current limit capability, the on time of the upper MOSFET, t
ON
, must be greater
than 400 ns (see Electrical Characteristics table). Therefore
V
O(min)
V
IN(max)
+
t
ON
T
SW
or
1
T
SW
+ f
SW
+
ȧ
ȧ
ȧ
ȧ
ȡ
Ȣ
ǒ
V
O(min)
V
IN(max)
Ǔ
T
ON
ȧ
ȧ
ȧ
ȧ
ȣ
Ȥ
Using 450 ns to provide margin,
f
SW
+
0.086
450 ns
+ 191 kHz
Since the oscillator can vary by 10%, decrease f
SW
, by 10%
f
SW
+ 0.9 191 kHz + 172 kHz
and therefore choose a frequency of 170 kHz.
3. Select ∆I
In this case ∆I is chosen so that the converter enters discontinuous mode at 20% of nominal load.
DI + I
O
2 0.2 + 8 2 0.2 + 3.2 A
(39)
(40)
(41)
(42)
(43)