Datasheet

8
LT1302/LT1302-5
APPLICATIONS INFORMATION
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and there is no low frequency component. The total ESR
is under 0.03. If a single 100µF aluminum electrolytic
capacitor is used instead, the converter mode-hops be-
tween current mode and Burst Mode due to high ESR,
causing the voltage comparator to trip as shown in Figure
5. The ripple voltage is now over 500mV
P-P
and contains
a low frequency component. Maximum allowable output
capacitor ESR can be calculated by the following formula:
ESR
VV
VA
MAX
OS OUT
REF
=
×
×1
where,
V
OS
= 15mV
V
REF
= 1.24V
500µs/DIV 1302 F04
V
OUT
50mV/DIV
AC COUPLED
I
LOAD
Figure 4. Low ESR Output Capacitor Results in Stable
Operation. Ripple Voltage is Under 30mV
P-P
500µs/DIV 1302 F05
V
OUT
200mV/DIV
AC COUPLED
I
LOAD
Figure 5. Inexpensive Electrolytic Capacitor Has High
ESR, Resulting in Mode-Hop, Ripple Voltage Amplitude Is
Over 500mV
P-P
and Includes Low Frequency Component
510mA
10mA
510mA
10mA
Input Capacitor
The input supply should be decoupled with a good quality
electrolytic capacitor close to the LT1302 to provide a
stable input supply. Long leads or traces from power
source to the switcher can have considerable impedance
at the LT1302’s switching frequency. The input capacitor
provides a low impedance at high frequency. A 0.1µF
ceramic capacitor is required right at the V
IN
pin. When the
input voltage can be above 5V, a 10/1µF decoupling
network for V
IN
is recommended as detailed in Figure 6.
This network is also recommended when driving a trans-
former.
Figure 6. A 10/1µF Decoupling Network at V
IN
Is
Recommended When Input Voltage Is Above 5V
++
V
IN
SW
PGND
1µF
LT1302
• • •
GND
47µF
TO
100µF
10
1302 F06
V
IN
> 5V
Table 2 lists capacitor vendors along with device types.
Table 2. Recommended Capacitors
VENDOR SERIES TYPE PHONE NO.
AVX TPS Surface Mount (803) 448-9411
Sanyo OS-CON Through Hole (619) 661-6835
Sprague 595D Surface Mount (603) 224-1961
Diode Selection
A 2A Schottky diode such as Motorola MBRS130LT3 has
been found to be the best available. Other choices include
1N5821 or MBRS130T3. Do not use “general purpose”
diodes such as 1N4001. They are much too slow for use
in switching regulator applications.