Datasheet

I
CIN_RMS
= I
OUT
D(1 - D)
I
CIN_RMS
= I
OUT
'I
L
2
12 I
OUT
2
1 - D +
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D
1
2
'I
L
R
ESR
V
OUT
= V
OUT_AVG
- 'V
OUT
= V
OUT_AVG
-
1
2
V
OUT_AVG
Time
V
REF
V
OUT
'V
OUT
R
FB1
= R
FB2
V
OUT
V
FB
- 1
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LM2696
SNVS375B –OCTOBER 2005–REVISED APRIL 2013 www.ti.com
FEEDBACK RESISTORS
The feedback resistors are used to scale the output voltage to the internal reference value such that the loop can
be regulated. The feedback resistors should not be made arbitrarily large as this creates a high impedance node
at the feedback pin that is more susceptible to noise. Typically, R
FB2
is on the order of 1 kΩ. To calculate the
value of R
FB1
, one may use the relationship:
Where
• V
FB
is the internal reference voltage that can be found in the ELECTRICAL CHARACTERISTICS table (1.254V
typical). (12)
The output voltage value can be set in a precise manner by taking into account the fact that the reference
voltage is regulating the bottom of the output ripple as opposed to the average value. This relationship is shown
in Figure 19.
Figure 19. Average and Ripple Output Voltages
It can be seen that the average output voltage is higher than the gained up reference by exactly half the output
voltage ripple. The output voltage may then be appended according to the voltage ripple. The appended V
OUT
term may be expressed using the relationship:
(13)
One should note that for high output voltages (>5V), a load of approximately 15 mA may be required for the
output voltage to reach the desired value.
INPUT CAPACITOR
Because PV
IN
is the power rail from which the output voltage is derived, the input capacitor is typically selected
according to the load current. In general, package size and ESR determine the current capacity of a capacitor. If
these criteria are met, there should be enough capacitance to prevent impedance interactions with the source. In
general, it is recommended to use a low ESR, high capacitance electrolytic and ceramic capacitor in parallel.
Using two capacitors in parallel ensures adequate capacitance and low ESR over the operating range. The
Sanyo MV-WX series electrolytic capacitors and a ceramic capacitor with X5R or X7R dielectric are an excellent
combination. To calculate the input capacitor RMS, one may use the following relationship:
(14)
that can be approximated by,
(15)
Typical values are 470 µF for the electrolytic capacitor and 0.1 µF for the ceramic capacitor.
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