LTC1707 LINER [Linear Technology], LTC1707 Datasheet - Page 9

• Manufacturer Part Number: LTC1707
• Description: High Efficiency Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
size or height requirements in the design. Always consult the
manufacturer if there is any question.
The selection of C
resistance (ESR). Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering. The output
ripple V
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. For the LTC1707, the general rule for
proper operation is:
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
the RMS current rating generally far exceeds the
I
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
RIPPLE(P-P)
C
OUT
V
OUT
L
= ripple current in the inductor. The output ripple
required ESR < 0.25
Figure 4. Setting the LTC1707 Output Voltage
OUT
requirement. Remember ESR is typically a
is determined by:
I ESR
L
OUT
LTC1707
U
is driven by the required effective series
GND
4
V
U
FB
fC
1
OUT
0.8V V
OUT
OUT
W
R1
R2
= output capacitance
1707 F04
8.5V
OUT
has been met,
L
U
increases
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:
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
increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 2.25 A current source charges up an external
capacitor C
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
and ends at the Figure 3 value (V
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:
Pulling the RUN/SS pin below 0.4V puts the LTC1707 into
a low quiescent current shutdown (I
be driven directly from logic as shown in Figure 5. Diode
V
t
DELAY
OUT
3.3V OR 5V
0 8 1
SS
2 25
0 7
Figure 5. RUN/SS Pin Interfacing
. When the voltage on RUN/SS reaches
V
D1
C
SS
A
RUN/SS
R
R
2
1
C
SS
Q
RUN/SS
< 15 A). This pin can
LTC1707
IN
RUN/SS
RUN/SS
by gradually
1.8V). The
C
SS
1707 F05
0.7V)
9
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