lt3430-1 Linear Technology Corporation, lt3430-1 Datasheet - Page 9

• Manufacturer Part Number: lt3430-1
• Description: High Voltage, 3a, 100khz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
tion should be used if resistors are increased beyond the
suggested values and short-circuit conditions occur with
high input voltage. High frequency pickup 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 47µH (10µH to 100µH for LT3430-1).
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 LT3430/LT3430-1
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
confl icting requirements.
Output Ripple Voltage
Figure 3 shows a comparison of output ripple voltage for
the LT3430/LT3430-1 using either a tantalum or ceramic
output capacitor. It can be seen from Figure 3 that output
ripple voltage can be signifi cantly reduced by using the
LT3430
V
C
Q2
GND
AMPLIFIER
ERROR
TO SYNC CIRCUIT
Figure 2. Frequency and Current Limit Foldback
TO FREQUENCY
SHIFTING
+
+
–
–
1.4V
1.2V
R3
1k
BUFFER
Q1
R4
2k
ceramic output capacitor; the signifi cant decrease in out-
put ripple voltage is due to the very low ESR of ceramic
capacitors.
Output ripple voltage is determined by ripple current (I
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 inductor).
The following equations will help in choosing the required
20mV/DIV
20mV/DIV
Figure 3. LT3430 Output Ripple Voltage Waveforms.
Ceramic vs Tantalum Output Capacitors
V
V
L = 22µH
IN
OUT
= 40V
V
= 5V
SW
FB
L1
R1
R2
LT3430/LT3430-1
+
2µs/DIV
3430 F02
C1
OUTPUT
5V
3430 F03
V
100µF CERAMIC
OUTPUT
CAPACITOR
V
100µF
TANTALUM
OUTPUT
CAPACITOR
OUT
OUT
USING
USING
0.08Ω
LP-P
34301fa
9
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