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LTC3852

3852f

APPLICATIONS INFORMATION

is given to ripple current ratings, temperature and long-

term reliability. A typical application will require several

to many aluminum electrolytic capacitors in parallel. A

combination of the above mentioned capacitors will often

result in maximizing performance and minimizing overall

cost. Other capacitor types include Nichicon PL series, NEC

Neocap, Panasonic SP and Sprague 595D series. Consult

manufacturers for other speciﬁc recommendations.

Like all components, capacitors are not ideal. Each

capacitor has its own beneﬁts and limitations. Combina-

tions of different capacitor types have proven to be a very

cost effective solution. Remember also to include high

frequency decoupling capacitors. They should be placed

as close as possible to the power pins of the load. Any

inductance present in the circuit board traces negates

their usefulness.

Setting Output Voltage

The LTC3852 output voltage is set by an external feedback

resistive divider carefully placed across the output,

as shown in Figure 7. The regulated output voltage is

determined by:

OUT = 0.8V1+

⎛

⎝⎜

⎞

⎠⎟

To improve the transient response, a feed-forward ca-

pacitor, CFF, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

LTC3852

VFB

VOUT

CFF

3852 F07

Figure 7. Settling Output Voltage

Fault Conditions: Current Limit and Current Foldback

The LTC3852 includes current foldback to help limit load

current when the output is shorted to ground. If the output

falls below 40% of its nominal output level, the maximum

sense voltage is progressively lowered from its maximum

programmed value to about 25% of the that value. Foldback

current limiting is disabled during soft-start or tracking.

Under short-circuit conditions with very low duty cycles,

the LTC3852 will begin cycle skipping in order to limit the

short-circuit current. In this situation the bottom MOSFET

will be dissipating most of the power but less than in normal

operation. The short-circuit ripple current is determined

by the minimum on-time tON(MIN) of the LTC3852 (≈90ns),

the input voltage and inductor value:

ΔI

L(SC) = tON(MIN) •

The resulting short-circuit current is:

SC =

1/4MaxV

SENSE

–

ΔI

L(SC)

Programming Switching Frequency

To set the switching frequency of the LTC3852, connect

a resistor, RFREQ, between FREQ/PLLFLTR and GND. The

relationship between the oscillator frequency and RFREQ

is shown in Figure 8. A 0.1μF bypass capacitor should be

connected in parallel with RFREQ.

RFREQ (kΩ)

250

300

400

450

500

750

600

100

120

3852 F08

350

650

700

550

140

160

Figure 8. Relationship Between Oscillator Frequency

and Resistor Connected Between FREQ/PLLFLTR and GND

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