Datasheet

Applications Information (Continued)
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,
supposea3msstart 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 kΩ so C can be calculated to be 2.2
nF. A 2.2 nF ceramic capacitor can be chosen to yield
approximatelya3msstart-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
voltage the output reached during the short circuit event. The
range of soft-start capacitors is therefore restricted to values
1nFto50nF.
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 µH inductor can be paired with a 220 µF (50 mΩ)
capacitor without degraded phase margin.
TABLE 1. Recommended L
O
and C
O
Values
V
OUT
(V) VIN (V)
L
O
(µH) C
O
(µF) C
ESR
(mΩ)
Min Max Min Max Min Max
0.8 5 4.7 6.8 120 220 70 100
0.8 3.3 4.7 4.7 150 220 50 100
1 5 4.7 6.8 120 220 70 100
1 3.3 4.7 4.7 150 220 50 100
1.2 5 4.7 6.8 120 220 70 100
1.2 3.3 4.7 4.7 120 220 60 100
1.5 5 4.7 6.8 120 220 70 100
1.5 3.3 4.7 4.7 120 220 60 100
1.8 5 4.7 6.8 120 220 70 120
1.8 3.3 4.7 4.7 100 220 70 120
2.5 5 4.7 6.8 120 220 70 150
2.5 3.3 4.7 4.7 100 220 80 150
3.0 5 4.7 6.8 120 220 70 150
3.0 3.3 4.7 4.7 100 220 80 150
3.3 5 4.7 6.8 120 220 70 150
LM2853
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