LT1956/LT1956-5
APPLICATIO S I FOR ATIO
LT1956
ERROR
AMPLIFIER
TO FREQUENCY
SHIFTING
1.4V
Q1
+
–
Q2
TO SYNC CIRCUIT
V
C
GND
Figure 2. Frequency and Current Limit Foldback
with high input voltage.
High frequency pickup will in-
crease 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 30µH. 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 LT1956 switch, which has a 1.5A 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 LT1956 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.
U
V
SW
L1
OUTPUT
5V
1.2V
R3
1k
R4
2k
BUFFER
R2
FB
R1
W
U U
+
C1
1956 F02
10mV/DIV
V
OUT
USING
22µF CERAMIC
OUTPUT
CAPACITOR
10mV/DIV
V
OUT
USING
100µF, 0.08Ω
TANTALUM
OUTPUT
CAPACITOR
V
IN
= 12V
V
OUT
= 5V
L = 15µH
1µs/DIV
1956 F03
Figure 3. LT1956 Output Ripple Voltage Waveforms.
Ceramic vs Tantalum Output Capacitors
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 importance,
the subsequent suggestions in Peak Inductor and Fault
Current and EMI will additionally help in the selection of
the inductor value.
1956f
9
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