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LTC3537

3537fa

APPLICATIONS INFORMATION

VINB > VOUTB OPERATION

The LTC3537 step-up converter will maintain voltage regu-

lation even when the input voltage is above the desired

output voltage. Note that the efﬁciency is much lower in this

mode, and the maximum output current capability will be

less. Refer to the Typical Performance Characteristics.

STEP-UP SHORT-CIRCUIT PROTECTION

The LTC3537 output disconnect feature provides output

short circuit protection. To reduce power dissipation under

short-circuit conditions, the peak switch current limit is

reduced to 400mA (typical).

SCHOTTKY DIODE

Although it is not required, adding a Schottky diode from

SW to VOUTB will improve efﬁciency by about 4%. Note

that this defeats the output disconnect and short-circuit

protection features.

PCB LAYOUT GUIDELINES

The high speed operation of the LTC3537 demands careful

attention to board layout. A careless layout will result in

reduced performance. Figure 1 shows the recommended

component placement. A large ground pin copper area

will help to lower the die temperature. A multilayer board

with a separate ground plane is ideal, but not absolutely

necessary.

COMPONENT SELECTION

Inductor Selection

The LTC3537 can utilize small surface mount chip induc-

tors due to its fast 2.2MHz switching frequency. Inductor

values between 1μH and 4.7μH are suitable for most ap-

plications. Larger values of inductance will allow slightly

greater output current capability (and lower the Burst

Mode threshold) by reducing the inductor ripple current.

Increasing the inductance above 10μH will increase size

while providing little improvement in output current capa-

bility. The minimum inductance value is given by:

INB(MIN) • VOUTB(MAX) − VINB(MIN)

()

Ripple

• V

OUTB(MAX)

where:

Ripple = Allowable inductor current ripple (amps

peak-peak)

VINB(MIN) = Minimum converter input voltage

VOUTB(MAX) = Maximum output voltage

The inductor current ripple is typically set for 20% to

40% of the maximum inductor current. High frequency

ferrite core inductor materials reduce frequency dependent

power losses compared to cheaper powdered iron types,

improving efﬁciency. The inductor should have low ESR

(series resistance of the windings) to reduce the I2R power

losses, and must be able to support the peak inductor

current without saturating. Molded chokes and some chip

inductors usually do not have enough core area to support

the peak inductor currents of 750mA seen on the LTC3537.

To minimize radiated noise, use a shielded inductor. See

Table 1 for suggested components and suppliers.

Figure 1

3537 F01

LBO

VOUTB

MODE

VINL

VOLDO

LBI

SGND

VINB

FBL

FBB

PGDB ENBST PGDL ENLDO

MULTIPLE VIAS

TO INNER GROUND LAYERS

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