Datasheet

9
LTC1707
size or height requirements in the design. Always consult the
manufacturer if there is any question.
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 LTC1707, 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. Remember ESR is typically a
direct function of the volume of the capacitor.
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 avail-
able 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, avail-
able in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POSCAP, KEMET T510
Figure 4. Setting the LTC1707 Output Voltage
RUN/SS
C
SS
D1
3.3V OR 5V
C
SS
RUN/SS
1707 F05
Figure 5. RUN/SS Pin Interfacing
and T495 series, Nichicon PL series and Sprague 593D
and 595D series. Consult the manufacturer for other
specific recommendations.
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.
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 LTC1707.
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 LTC1707 begins operating. As the voltage on
RUN/SS continues to ramp from 0.7V to 1.8V, the inter-
nal 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 LTC1707 into
a low quiescent current shutdown (I
Q
< 15µA). This pin can
be driven directly from logic as shown in Figure 5. Diode
0.8V ≤ V
OUT
≤ 8.5V
R2
R1
1707 F04
V
FB
GND
LTC1707
APPLICATIO S I FOR ATIO
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