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LTC7812

7812fc

For more information www.linear.com/LTC7812

APPLICATIONS INFORMATION

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet

size or height requirements in the design. Due to the high

operating frequency of the LTC7812, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

A small (0.1μF to 1μF) bypass capacitor between the chip

VIN pin and ground, placed close to the LTC7812, is also

suggested. A small (≤10Ω) resistor placed between CIN

(C1) and the VIN pin provides further isolation.

The selection of COUT is driven by the effective series

resistance (ESR). Typically, once the ESR requirement

is satisfied, the capacitance is adequate for filtering. The

output ripple (ΔVOUT) is approximated by:

DVOUT ≈ DIL ESR +

8 • f • COUT

⎛

⎝⎜

⎞

⎠⎟

where f is the operating frequency, COUT is the output

capacitance and ΔIL is the ripple current in the inductor.

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage.

Setting Output Voltage

The LTC7812 output voltages are each set by an external

feedback resistor divider carefully placed across the out-

put, as shown in Figure 3. The regulated output voltages

are determined by:

VOUT,BUCK = 0.8V 1+

⎛

⎝

⎜

⎞

⎠

⎟

VOUT,BOOST = 1.2V 1+

⎛

⎝

⎜

⎞

⎠

⎟

To improve the frequency response, a feedforward 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.

Tracking and Soft-Start (TRACK/SS1, SS2 Pins)

The start-up of each VOUT is controlled by the voltage on

the respective TRACK/SS pin (TRACK/SS1 for channel 1,

SS2 for channel 2). When the voltage on the TRACK/SS

pin is less than the internal 0.8V reference (1.2V reference

for the boost channel), the LTC7812 regulates the VFB pin

voltage to the voltage on the TRACK/SS pin instead of the

internal reference. Likewise, the TRACK/SS1 pin for the

buck channel can be used to program an external soft-

start function or to allow VOUT to track another supply

during start-up.

Figure 3. Setting Output Voltage

7812 F03

1/2 LTC7812

VFB

CFF

VOUT

Figure 4. Using the TRACK/SS Pin to Program Soft-Start

Soft-start is enabled by simply connecting a capacitor

from the TRACK/SS pin to ground, as shown in Figure 4.

An internal 5µA current source charges the capacitor,

providing a linear ramping voltage at the TRACK/SS pin.

The LTC7812 will regulate the VFB pin (and hence VOUT)

according to the voltage on the TRACK/SS pin, allowing

VOUT to rise smoothly from 0V to its final regulated value.

The total soft-start time will be approximately:

tSS_BUCK = CSS •

0.8V

5µA

tSS_BOOST = CSS •

1.2V

5µA

7812 F04

LTC7812

TRACK/SS

SGND

CSS

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