STEP-DOWN, 600 kHz PWM CONTROL or PWM/PFM SWITCHABLE SWITCHING REGULATOR CONTROLLER
Rev.4.0_00
S-8540/8541 Series
Seiko Instruments Inc. 21
R
A
, R
B
must be R
A
+ R
B
≤ 2 MΩ and the ratio of R
A
to R
B
should be set so that the FB pin is 1.0 V. Add a
capacitor (C
FB
) in parallel to R
A
to prevent unstable operation like output oscillation.
Set the C
FB
so that f = 1/(2 × πC
FB
× R
A
) is 0.1 to 20 kHz (normally 10 kHz).
e.g. When V
OUT
= 3.0 V, R
A
= 200 kΩ, R
B
= 100 kΩ, then C
FB
= 100 pF.
The precision of output voltage (V
OUT
) determined by R
A
, R
B
is affected by the precision of the voltage at
the FB pin (1 V
± 2.0%), the precision of R
A
and R
B
, current input to the FB pin, and IC power supply
voltage V
DD
.
Suppose that the FB pin input current is 0 nA, and that the maximum absolute values of the external
resistors R
A
and R
B
are R
A
max. and R
B
max, and the minimum absolute values of the external resistors
R
A
and R
B
are R
A
min. and R
B
min., and that the output voltage shift due to the V
DD
voltage
dependency is
ΔV, the minimum value V
OUT
min. and maximum value V
OUT
max. of the output voltage
V
OUT
variation is calculated by the following formula:
V
OUT
min. = (1 +
max.R
min.R
B
A
) × 0.98 − ΔV [V]
V
OUT
max. = (1 +
Rmax.
Rmin.
A
B
) × 1.02 + ΔV [V]
The precision of the output voltage V
OUT
cannot be made lower than the precision of the IC output voltage
without adjustment of external resistors R
A
and R
B
. The lower the R
A
/R
B
, the less it is affected by the
absolute value precision of the external resistors R
A
and R
B
. The lower the R
A
and R
B
, the less it is
affected by the FB pin input current.
To suppress the influence of FB pin input current on the variation of output voltage V
OUT
, the external
resistor R
B
value must be made sufficiently lower than the input impedance of the FB pin, 1 V/50 nA =
20 M
Ω max.
Waste current flows through external resistors R
A
and R
B
. When it is not a negligible value with respect
to load current in actual use, the efficiency decreases. The R
A
and R
B
values of the external resistors
must therefore be made sufficiently high.
Evaluation of the influence of the noise is needed in the actual condition If the R
A
and R
B
values of
resistors are high (1 M
Ω or higher) since they are susceptible to external noise.
The output voltage V
OUT
precision and the waste current are in a trade-off relation. They must be
considered according to application requests.
