8

LTC1626

To avoid overheating, the output capacitor must be sized

to handle the ripple current generated by the inductor. The

worst-case RMS ripple current in the output capacitor is

given by:

requirement.

In some surface mount applications, multiple capacitors

may have to be paralleled to meet the capacitance, ESR or

RMS current handling requirement of the application.

Aluminum electrolyte 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 tantalums, available

in case heights ranging from 2mm to 4mm. Other capaci-

tor types include Sanyo OS-CON, Nichicon PL series and

Sprague 595D series. Consult the manufacturer for other

specific recommendations.

output ripple at low frequencies will be large enough to trip

the voltage comparator. This causes Burst Mode opera-

tion to be activated when the LTC1626 would normally be

in continuous mode operation. The effect will be most

at higher frequencies.

Low-Battery Detection

The low-battery detector senses the input voltage through

an external resistive divider. This divided voltage connects

to the (–) input of a voltage comparator (LBI) and is

compared to an internal 1.25V reference voltage. Neglect-

ing LBI input bias current, the following expression is used

for setting the trip voltage threshold:

V

R

R

LB TRIP

_

.

=+









125 1

4

3

The LBO is an N-channel open drain that goes low when

the battery voltage drops below the threshold voltage. In

shutdown, the comparator is disabled and LBO is in the

high impedance state. Figure 4 is a schematic diagram

detailing the low-battery comparator connection and op-

eration.

LBI

R4

1%

R3

1%

0.01

µF

1.25V

LBO

1626 F04

LTC1626

Figure 4. Low-Battery Comparator

Setting the Output Voltage

The LTC1626 develops a 1.25V reference voltage between

Figure 5. By selecting resistor R1, a constant current is

caused to flow through R1 and R2 which sets the desired

output voltage. The regulated output voltage is deter-

mined by:

V

R

R









125 1

2

1

.

R1 should be

≤ 10k to ensure that sufficient current flows

through the divider to maintain accuracy and to provide a

minimum load for the regulator output at elevated

temperatures. (See Switch Leakage Current curve in Typi-

cal Performance Characteristics section.)

To prevent stray pickup, a 100pF capacitor is suggested

across R1, located close to the LTC1626.

100pF

R2

1%

R1

10k

1%

SGND

1626 F05

LTC1626

Figure 5. Setting the Output Voltage