Datasheet
9
LT3431
sn3431 3431fs
10mV/DIV
will increase and the protection accorded by frequency
and current foldback will decrease.
CHOOSING THE INDUCTOR
For most applications, the output inductor will fall into the
range of 5µH to 33µH. Lower values are chosen to reduce
physical size of the inductor. Higher values allow more
output current because they reduce peak current seen by
the LT3431 switch, which has a 3A limit. Higher values
also reduce output ripple voltage.
When choosing an inductor you will need to consider
output ripple voltage, maximum load current, peak induc-
tor current and fault current in the inductor. In addition,
other factors such as core and copper losses, allowable
component height, EMI, saturation and cost should also
be considered. The following procedure is suggested as a
way of handling these somewhat complicated and con-
flicting requirements.
Output Ripple Voltage
Figure 3 shows a comparison of output ripple voltage for
the LT3431 using either a tantalum or ceramic output
capacitor. It can be seen from Figure 3 that output ripple
voltage can be significantly reduced by using the ceramic
output capacitor; the significant decrease in output ripple
voltage is due to the very low ESR of ceramic capacitors.
–
+
1.2V
BUFFER
V
SW
L1
V
C
GND
TO SYNC CIRCUIT
3431 F02
TO FREQUENCY
SHIFTING
R3
1k
R4
2k
R1
C1
R2
OUTPUT
5V
ERROR
AMPLIFIER
FB
1.4V
Q1
LT3431
Q2
+
Figure 2. Frequency and Current Limit Foldback
APPLICATIO S I FOR ATIO
WUUU
Output ripple voltage is determined by ripple current
(I
LP-P
) through the inductor and the high frequency
impedance of the output capacitor. At high frequencies,
the impedance of the tantalum capacitor is dominated by
its effective series resistance (ESR).
Tantalum Output Capacitor
The typical method for reducing output ripple voltage
when using a tantalum output capacitor is to increase the
inductor value (to reduce the ripple current in the induc-
tor). The following equations will help in choosing the
required inductor value to achieve a desirable output
ripple voltage level. If output ripple voltage is of less
Figure 3. LT3431 Output Ripple Voltage Waveforms.
Ceramic vs Tantalum Output Capacitors
1µs/DIV
V
OUT
USING
47µF CERAMIC
OUTPUT
CAPACITOR
V
OUT
USING
100µF, 0.08Ω
TANTALUM
OUTPUT
CAPACITOR
V
IN
= 12V 3431 F03
V
OUT
= 5V
L = 10µH