LM2853
Applications Information
(Continued)
voltage the output reached during the short circuit event. The
range of soft-start capacitors is therefore restricted to values
1 nF to 50 nF.
COMPENSATION
The LM2853 provides a highly integrated solution to power
supply design. The compensation of the LM2853, which is
type-three, is included on-chip. The benefit of integrated
compensation is straight-forward, simple power supply de-
sign. Since the output filter capacitor and inductor values
impact the compensation of the control loop, the range of L
O
,
C
O
and C
ESR
values is restricted in order to ensure stability.
OUTPUT FILTER VALUES
Table 1
details the recommended inductor and capacitor
ranges for the LM2853 that are suggested for various typical
output voltages. Values slightly different than those recom-
mended may be used, however the phase margin of the
power supply may be degraded. For best performance when
output voltage ripple is a concern, ESR values near the
minimum of the recommended range should be paired with
capacitance values near the maximum. If a minimum output
voltage ripple solution from a 5V input voltage is desired, a
6.8 碌H inductor can be paired with a 220 碌F (50 m鈩?
capacitor without degraded phase margin.
Since it is undesirable for the power supply to start up in
current limit, a soft-start capacitor must be chosen to force
the LM2853 to start up in a more controlled fashion based on
the charging of the soft-start capacitance. In this example,
suppose a 3 ms start time is desired. Three time constants
are required for charging the soft-start capacitor to 95% of
the final reference voltage. So in this case RC = 1 ms. The
DAC resistor, R, is 450 k鈩?so C can be calculated to be 2.2
nF. A 2.2 nF ceramic capacitor can be chosen to yield
approximately a 3 ms start-up time.
SOFT-START CAPACITOR (C
SS
) AND FAULT
CONDITIONS
Various fault conditions such as short circuit and UVLO of
the LM2853 activate internal circuitry designed to control the
voltage on the soft-start capacitor. For example, during a
short circuit current limit event, the output voltage typically
falls to a low voltage. During this time, the soft-start voltage
is forced to track the output so that once the short is re-
moved, the LM2853 can restart gracefully from whatever
TABLE 1. Recommended L
O
and C
O
Values
L
O
(碌H)
V
OUT
(V)
0.8
0.8
1
1
1.2
1.2
1.5
1.5
1.8
1.8
2.5
2.5
3.0
3.0
3.3
VIN (V)
5
3.3
5
3.3
5
3.3
5
3.3
5
3.3
5
3.3
5
3.3
5
Min
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
Max
6.8
4.7
6.8
4.7
6.8
4.7
6.8
4.7
6.8
4.7
6.8
4.7
6.8
4.7
6.8
Min
120
150
120
150
120
120
120
120
120
100
120
100
120
100
120
C
O
(碌F)
Max
220
220
220
220
220
220
220
220
220
220
220
220
220
220
220
Min
70
50
70
50
70
60
70
60
70
70
70
80
70
80
70
C
ESR
(m鈩?
Max
100
100
100
100
100
100
100
100
120
120
150
150
150
150
150
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