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LTC3630

3630fb

APPLICATIONS INFORMATION

The basic LTC3630 application circuit is shown on the front

page of the data sheet. External component selection is

determined by the maximum load current requirement and

begins with the selection of the peak current programming

resistor, RISET.TheinductorvalueLcanthenbedetermined,

followed by capacitors CIN and COUT.

Peak Current Resistor Selection

The peak current comparator has a guaranteed maximum

current limit of 1A (1.2A typical), which guarantees a

maximum average current of 500mA. For applications

that demand less current, the peak current threshold can

be reduced to as little as 100mA (120mA typical). This

lower peak current allows the use of lower value, smaller

components (input capacitor, output capacitor, and induc-

tor), resulting in lower input supply ripple and a smaller

overall DC/DC converter.

The threshold can be easily programmed using a resis-

tor (RISET) between the ISET pin and ground. The voltage

generated on the ISET pin by RISET and the internal 5μA

current source sets the peak current. The voltage on the

ISET pin is internally limited within the range of 0.1V to

1.0V. The value of resistor for a particular peak current can

be selected by using Figure 2 or the following equation:

RISET = IPEAK • 0.2 • 106

where 100mA < IPEAK < 1A.

The internal 5μA current source is reduced to 1μA in sleep

mode to maximize efficiency and to facilitate a trade-off

between efficiency and light load output voltage ripple, as

described in the CISET Selection section of the Applica-

tions Information. For maximum efficiency, minimize the

capacitance on the ISET pin and place the RISET resistor

as close to the pin as possible.

The typical peak current is internally limited to be within the

range of 120mA to 1.2A. Shorting the ISET pin to ground

programs the current limit to 120mA, and leaving it float

sets the current limit to the maximum value of 1.2A. When

selecting this resistor value, be aware that the maximum

average output current for this architecture is limited to

half of the peak current. Therefore, be sure to select a value

that sets the peak current with enough margin to provide

adequate load current under all conditions. Selecting the

peak current to be 2.2 times greater than the maximum

load current is a good starting point for most applications.

Inductor Selection

The inductor, input voltage, output voltage, and peak cur-

rent determine the switching frequency during a burst

cycle of the LTC3630. For a given input voltage, output

voltage, and peak current, the inductor value sets the

switching frequency during a burst cycle when the output

is in regulation. Generally, switching between 50kHz and

250kHz yields high efficiency, and 200kHz is a good first

choice for many applications. The inductor value can be

determined by the following equation:

VOUT

f•IPEAK

⎛

⎝

⎜

⎞

⎠

⎟ •1–

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

The variation in switching frequency during a burst cycle

with input voltage and inductance is shown in Figure 3. For

lower values of IPEAK, multiply the frequency in Figure 3

by 1.2A/IPEAK.

An additional constraint on the inductor value is the

LTC3630’s 150ns minimum on-time of the high side switch.

Therefore, in order to keep the current in the inductor

Figure 2. RISET Selection

MAXIMUM LOAD CURRENT (mA)

180

200

220

150

250 300 350

3630 F02

140

100

160

120

100

200

400 450 500

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