LTC1872
8
1872fa
For more information
www.linear.com/LTC1872
applicaTions inForMaTion
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the inductor.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest ESR (size)
product of any aluminum electrolytic at a somewhat higher
price. The output capacitor RMS current is approximately
equal to:
I
PK
•
DC
−
DC
2
where I
PK
is the peak inductor current and DC is the switch
duty cycle.
When using electrolytic output capacitors, if the ripple and
ESR requirements are met, there is likely to be far more
capacitance than required.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. An excellent choice of
tantalum capacitors is the AVX TPS and KEMET T510
series of surface mount tantalum capacitors. Also,
ceramic capacitors in X5R pr X7R dielectrics offer excel-
lent performance.
Low Supply Operation
Although the LTC1872 can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V
IN
decreases below 3V. Figure 3 shows
the amount of change as the supply is reduced down to
2V. Also shown in Figure 3 is the effect of V
IN
on V
REF
as
V
IN
goes below 2.3V.
Setting Output Voltage
The LTC1872 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 4).
By selecting resistor R1, a constant current is caused to
flow through R1 and R2 to set the overall output voltage.
The regulated output voltage is determined by:
V
OUT
=
0.8V 1
+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
INPUT VOLTAGE (V)
2.0
NORMALIZED VOLTAGE (%)
105
100
95
90
85
80
75
2.2
2.4
2.6
2.8
1872 F03
3.0
V
REF
V
ITH
Figure 4. Setting Output Voltage
Figure 3. Line Regulation of V
REF
and V
ITH
3
V
FB
V
OUT
LTC1872
R1
1872 F04
R2
