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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

voltageinthismode.TyingVPRG2toINTVCCprogramsthe

boost output to 12V, whereas floating VPRG2 programs

the output to 10V.

7813 F04a

LTC7813

VFB2

VPRG2

GND

CFF

VOUT2

(4a) Setting Boost Output Using External Resistors

7813 F04b

LTC7813

(4b) Setting Boost to Fixed 12V/10V Output

VFB2

VPRG2

INTVCC/FLOAT

COUT

VOUT2

12V/10V

Figure 4. Setting Boost Output Voltage

RUN Pins

The LTC7813 is enabled using the RUN1 and RUN2 pins.

The RUN pins have a rising threshold of 1.275V with

75mV of hysteresis. Pulling a RUN pin below 1.2V shuts

down the main control loop for that channel. Pulling all

three RUN pins below 0.7V disables the controllers and

most internal circuits, including the DRVCC and INTVCC

LDOs. In this state, the LTC7813 draws only 3.6µA of

quiescent current.

Releasing a RUN pin allows a small 150nA internal current

to pull up the pin to enable that controller. Because of

condensation or other small board leakage pulling the pin

down,itisrecommendedtheRUNpinsbeexternallypulled

up or driven directly by logic. Each RUN pin can tolerate

up to 65V (absolute maximum), so it can be conveniently

tied to VBIAS in always-on applications where one or more

controllersareenabledcontinuouslyandnevershutdown.

The RUN pins can be implemented as a UVLO by con-

necting them to the output of an external resistor divider

network off VBIAS, as shown in Figure 5.

TherisingandfallingUVLOthresholdsarecalculatedusing

the RUN pin thresholds and pull-up current:

VUVLO(RISING) =1.275V 1+

⎛

⎝

⎜

⎞

⎠

⎟–150nA •RB

VUVLO(FALLING) =1.20V 1+

⎛

⎝

⎜

⎞

⎠

⎟–150nA •RB

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

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

the 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 LTC7813 regulates the VFB pin

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

the internal reference. The TRACK/SS pin can be used to

program an external soft-start function or to allow VOUT

to track another supply during start-up.

Soft-start is enabled by simply connecting a capacitor

from the TRACK/SS pin to ground, as shown in Figure 6.

An internal 10μA current source charges the capacitor,

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

The LTC7813 will regulate its feedback voltage (and hence

VOUT) according to the voltage on the TRACK/SS pin, al-

lowing VOUT to rise smoothly from 0V (VIN for the boost)

7813 F05

LTC7813

RUN

VBIAS

Figure 5. Using the RUN Pins as a UVLO

7813 F06

LTC7813

TRACK/SS

GND

CSS

Figure 6. Using the TRACK/SS Pin to Program Soft-Start

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