Applications Information: continued
where VSAT = Rds(on) ´ IOUT max. and Rds(on)is the value at
TJ 100ûC.
5) Output capacitor
If VF = 0.60V and VSAT = 0.60V then the above equation
becomes:
The output capacitor and the inductor form a low pass fil-
ter. The output capacitor should have a low ESL and ESR.
Low impedance aluminum electrolytic, tantalum or organ-
ic semiconductor capacitors are a good choice for an out-
put capacitor. Low impedance aluminum are less expen-
sive. Solid tantalum chip capacitors are available from a
number of suppliers and are the best choice for surface
mount applications.
5.6
9
= 0.62
DMAX
=
5.6
13.8
= 0.40
DMIN
=
The output capacitor limits the output ripple voltage. The
CS51031 needs a maximum of 20mV of output ripple for
the feedback comparator to change state. If we assume that
all the inductor ripple current flows through the output
capacitor and that it is an ideal capacitor (i.e. zero ESR), the
minimum capacitance needed to limit the output ripple to
50mV peak to peak is given by:
2) Switching frequency and on and off time calculations
Given that fSW = 200kHz and DMAX = 0.80
1
T =
= 5µs
fSW
ON(max) = T ´ DMAX = 5µs ´ 0.62 @ 3µs
T
ÆI
0.6A
C =
=
= 7.5µF
8 ´ (200 ´ 103Hz) ´ (50 ´ 10-3V)
8 ´ fSW ´ ÆV
TON(min) = T ´ DMIN = 5µs ´ 0.40 = 2µs
TOFF(max) = TON(min) = 5µs - 2µs = 3µs
The minimum ESR needed to limit the output voltage rip-
ple to 50mV peak to peak is:
50 ´ 10-3
0.6A
ÆV
ÆI
3) Oscillator Capacitor Selection
ESR =
=
= 83m½
The switching frequency is set by COSC, whose value is
given by:
The output capacitor should be chosen so that its ESR is
less than 83m½.
95 ´ 10-6
COSC in pF =
During the minimum off time, the ripple current is 0.4A
and the output voltage ripple will be:
30 ´ 10 3
2
Fsw
3 ´ 10 6
Fsw
1+
-
Fsw
(
)
(
)
´
´ 0.4 = 33mV.
ÆV=ESR ÆI = 83m½
4) Inductor selection
6) VFB divider
VOUT = 1.25V
The inductor value is chosen for continuous mode opera-
tion down to 0.3Amps.
R1 + R2
R2
R1
R2
= 1.25V
+ 1
(
)
(
)
The ripple current ÆI = 2 ´ IOUTmin = 2 ´ 0.3A = 0.6A.
The input bias current to the comparator is 4µA. The resis-
tor divider current should be considerably higher than this
to ensure that there is sufficient bias current. If we choose
the divider current to be at least 250 times the bias current
this permits a divider current of 1mA and simplifies the
calculations.
(VOUT + VD) ´ TOFF(max)
5.6V ´ 3µs
Lmin
=
=
=28µH
ÆI
0.6A
This is the minimum value of inductor to keep the ripple
current to <0.6A during normal operation.
5V
A smaller inductor will result in larger ripple current.
Ripple current at a minimum off time is
= R1 + R2 = 5k½
1mA
(VOUT + VF) ´ TOFF(min)
5.6V ´ 2µs
Let R2 = 1K
ÆI =
=
=0.4A
LMIN
28µH
Rearranging the divider equation gives:
VOUT
1.25
5V
1.25
R1 = R2
- 1 = 1k½
- 1 = 3k½
)
)
(
(
The core must not saturate with the maximum expected
current, here given by:
IMAX = IOUT + ÆI/2 = 3A+0.4A/2 = 3.2A
6
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