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LTC3548

3548fc

This decrease causes the error ampliﬁer to increase the

ITH voltage until the average inductor current matches the

new load current.

The main control loop is shut down by pulling the RUN

pin to ground.

Low Current Operation

By selecting MODE/SYNC (Pin 6), two modes are available

to control the operation of the LTC3548 at low currents. Both

modes automatically switch from continuous operation to

the selected mode when the load current is low.

To optimize efﬁciency, the Burst Mode operation can be

selected. When the load is relatively light, the LTC3548

automatically switches into Burst Mode operation, in which

the PMOS switch operates intermittently based on load

demand with a ﬁxed peak inductor current. By running

cycles periodically, the switching losses which are domi-

nated by the gate charge losses of the power MOSFETs

are minimized. The main control loop is interrupted when

the output voltage reaches the desired regulated value. A

voltage comparator trips when ITH is below 0.35V, shutting

off the switch and reducing the power. The output capaci-

tor and the inductor supply the power to the load until ITH

exceeds 0.65V, turning on the switch and the main control

loop which starts another cycle.

For lower ripple noise at low currents, the pulse-skipping

mode can be used. In this mode, the LTC3548 continues to

switch at a constant frequency down to very low currents,

where it will begin skipping pulses. The efﬁciency in pulse-

skipping mode can be improved slightly by connecting

the SW node to the MODE/SYNC input which reduces the

clock frequency by approximately 30%.

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases to 100% which

is the dropout condition. In dropout, the PMOS switch is

turned on continuously with the output voltage being equal

to the input voltage minus the voltage drops across the

internal P-channel MOSFET and the inductor.

An important design consideration is that the RDS(ON)

of the P-channel switch increases with decreasing input

supply voltage (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipation

when the LTC3548 is used at 100% duty cycle with low

input voltage (see Thermal Considerations in the Applica-

tions Information section).

Low Supply Operation

To prevent unstable operation, the LTC3548 incorporates

an undervoltage lockout circuit which shuts down the part

when the input voltage drops below about 1.65V.

A general LTC3548 application circuit is shown in

Figure 2. External component selection is driven by the

load requirement, and begins with the selection of the

inductor L. Once the inductor is chosen, CIN and COUT

can be selected.

Inductor Selection

Although the inductor does not inﬂuence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current ΔIL decreases with

higher inductance and increases with higher VIN or VOUT:

ΔI

L =

OUT

O •L

•1–

OUT

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

ΔIL = 0.3 • IOUT(MAX), where IOUT(MAX) is 800mA for channel

1 and 400mA for channel 2. The largest ripple current

ΔIL occurs at the maximum input voltage. To guarantee

that the ripple current stays below a speciﬁed maximum,

the inductor value should be chosen according to the

following equation:

≥

OUT

O • ΔIL

•1–

OUT

IN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

OPERATION

APPLICATIONS INFORMATION

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