LM2853-1.8EVAL National Semiconductor, LM2853-1.8EVAL Datasheet - Page 8
![no-image](/images/manufacturer_photos/0/4/469/national_semiconductor_sml.jpg)
LM2853-1.8EVAL
Manufacturer Part Number
LM2853-1.8EVAL
Description
BOARD EVAL LM2853-1.8
Manufacturer
National Semiconductor
Series
PowerWise®, SIMPLE SWITCHER®r
Specifications of LM2853-1.8EVAL
Main Purpose
DC/DC, Step Down
Outputs And Type
1, Non-Isolated
Voltage - Output
1.8V
Current - Output
3A
Voltage - Input
3 ~ 5.5V
Regulator Topology
Buck
Frequency - Switching
550kHz
Board Type
Fully Populated
Utilized Ic / Part
LM2853
Lead Free Status / RoHS Status
Not applicable / Not applicable
Power - Output
-
www.national.com
Applications Information
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 kΩ 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
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
V
OUT
0.8
0.8
1.2
1.2
1.5
1.5
1.8
1.8
2.5
2.5
3.0
3.0
3.3
1
1
(V)
VIN (V)
3.3
3.3
3.3
3.3
3.3
3.3
3.3
5
5
5
5
5
5
5
5
SS
) AND FAULT
Min
TABLE 1. Recommended L
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
(Continued)
L
O
(µH)
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
8
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
C
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.
O
O
Min
120
150
120
150
120
120
120
120
120
100
120
100
120
100
120
and C
and C
C
O
ESR
O
(µF)
Values
values is restricted in order to ensure stability.
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
(mΩ)
Max
100
100
100
100
100
100
100
100
120
120
150
150
150
150
150
O
,