9

LTC1707

size or height requirements in the design. Always consult the

manufacturer if there is any question.

resistance (ESR). Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering. The output

ripple

∆∆

V

I ESR

fC

OUT

L

OUT

≅+









1

4

and

is highest at maximum input voltage since

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

the RMS current rating generally far exceeds the

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

D1

3.3V OR 5V

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









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.

increasing the internal current limit. Power supply

sequencing can also be accomplished using this pin.

An internal 2.25

µA current source charges up an external

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

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

be driven directly from logic as shown in Figure 5. Diode

0.8V

R2

R1

1707 F04

GND

LTC1707

APPLICATIO S I FOR ATIO