Datasheet

9
LTC1627
Figure 4. Setting the LTC1627 Output Voltage
0.8V ≤ V
OUT
≤ 8.5V
R2
R1
1627 F04
V
FB
GND
LTC1627
Run/Soft-Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1627.
Soft-start reduces surge currents from V
IN
by gradually
increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.25µA current source charges up an external
capacitor C
SS
. When the voltage on RUN/SS reaches 0.7V
the LTC1627 begins operating. As the voltage on RUN/SS
continues to ramp from 0.7V to 1.8V, the internal current
limit is also ramped at a proportional linear rate. The
current limit begins at 25mA (at V
RUN/SS
≤ 0.7V) and ends
at the Figure 3 value (V
RUN/SS
≈ 1.8V). The output current
thus ramps up slowly, charging the output capacitor. If
RUN/SS has been pulled all the way to ground, there will
be a delay before the current starts increasing and is given
by:
t
C
A
DELAY
SS
=
07
225
.
.µ
Pulling the RUN/SS pin below 0.4V puts the LTC1627 into
a low quiescent current shutdown (I
Q
< 15µA). This pin can
be driven directly from logic as shown in Figure 5. Diode
D1 in Figure 5 reduces the start delay but allows C
SS
to
ramp up slowly providing the soft-start function. This
diode can be deleted if soft-start is not needed.
RUN/SS
C
SS
D1
3.3V OR 5V
C
SS
RUN/SS
1627 F05
Figure 5. RUN/SS Pin Interfacing
The selection of C
OUT
is driven by the required effective series
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The output
ripple ∆V
OUT
is determined by:
∆∆V I ESR
fC
OUT L
OUT
≅+






1
8
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I
L
increases
with input voltage. For the LTC1627, the general rule for
proper operation is:
C
OUT
required ESR < 0.25Ω
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
ratio of any aluminum electrolytic at a somewhat higher
price. Once the ESR requirement for C
OUT
has been met,
the RMS current rating generally far exceeds the
I
RIPPLE(P-P)
requirement.
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalum, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo POSCAP, KEMET T510 and T495 series,
Nichicon PL series and Sprague 593D and 595D series.
Consult the manufacturer for other specific recommenda-
tions.
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VV
R
R
OUT
=+






08 1
2
1
.
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 4.
APPLICATIO S I FOR ATIO
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