LTC3104 datasheet(15/26 Pages) LINER

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LTC3630

3630fb

APPLICATIONS INFORMATION

The resistive divider allows the VFB pin to sense a fraction

of the output voltage as shown in Figure 6. The output

voltage can range from 0.8V to VIN. Be careful to keep the

divider resistors very close to the VFB pin to minimize the

trace length and noise pick-up on the sensitive VFB signal.

RUN Pin and External Input Undervoltage Lockout

The RUN pin has two different threshold voltage levels.

Pulling the RUN pin below 0.7V puts the LTC3630 into a

low quiescent current shutdown mode (IQ ~ 5μA). When

the RUN pin is greater than 1.21V, the controller is enabled.

Figure 8 shows examples of configurations for driving the

RUN pin from logic.

VFB

VOUT

3630 F06

0.8V

VPRG1

VPRG2

LTC3630

Figure 6. Setting the Output Voltage with External Resistors

4.2M

3630 F07

VOUT

800k

0.8V

VFB

VPRG1

VPRG2

LTC3630

Figure 7. Setting the Output Voltage with

External and Internal Resistors

RUN

SUPPLY

LTC3630

RUN

3630 F08

LTC3630

Figure 8. RUN Pin Interface to Logic

Figure 9. Adjustable UV Lockout

RUN

SLEEP, ACTIVE: 2μA

SHUTDOWN: 0μA

3630 F09

VIN

LTC3630

To minimize the no-load supply current, resistor values in

the megohm range may be used; however, large resistor

values should be used with caution. The feedback divider

is the only load current when in shutdown. If PCB leakage

current to the output node or switch node exceeds the load

current, the output voltage will be pulled up. In normal

operation, this is generally a minor concern since the load

current is much greater than the leakage.

To avoid excessively large values of R1 in high output volt-

age applications (VOUT ≥ 10V), a combination of external

and internal resistors can be used to set the output volt-

age. This has an additional benefit of increasing the noise

immunity on the VFB pin. Figure 7 shows the LTC3630

with the VFB pin configured for a 5V fixed output with an

external divider to generate a higher output voltage. The

internal 5M resistance appears in parallel with R2, and the

value of R2 must be adjusted accordingly. R2 should be

chosen to be less than 200k to keep the output voltage

variation less than 1% due to the tolerance of the LTC3630’s

internal resistor.

The RUN pin can alternatively be configured as a precise

undervoltage (UVLO) lockout on the VIN supply with a

resistive divider from VIN to ground. A simple resistive

divider can be used as shown in Figure 9 to meet specific

VIN voltage requirements.

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