LTC3894 datasheet(20/32 Pages) LINER

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LTC3894

Rev 0

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APPLICATIONS INFORMATION

The continuous/discontinuous boundary is inversely

proportional to the inductor value. Therefore, if required,

IOUT(CDB) can be reduced by increasing the inductor value.

RUN Pin and VIN Overvoltage/Undervoltage Lockout

The LTC3894 is enabled using the RUN pin. It has a rising

threshold of 1.24V with 100mV of hysteresis. Pulling the

RUN pin below 1.12V shuts down the main control loop.

Pulling it below 0.8V disables the controller and most

internal circuits. In this state the LTC3894 draws only 7μA

of quiescent current.

The RUN pin is high impedance and must be externally

pulled up/down or driven directly by logic. The RUN pin

can tolerate up to 150V (absolute maximum), so it can be

conveniently tied to VIN in always-on applications where

the controller is enabled continuously and never shut

down.

The RUN and OVLO pins can alternatively be configured as

adjustable undervoltage (UVLO) and overvoltage (OVLO)

lockouts on the VIN supply with a resistor divider from VIN

to ground. A simple resistor divider can be used as shown

in Figure 6 to meet specific VIN voltage requirements.

The current that flows through the R3-R4-R5 divider will

directly add to the shutdown, sleep, and active current

of the LTC3894, and care should be taken to minimize

the impact of this current on the overall efficiency of the

application circuit. Resistor values in the megaohm range

may be required to keep the impact on quiescent shut-

down and sleep currents low. To pick resistor values, the

sum total of R3 + R4 + R5 (RTOTAL) should be chosen

first based on the allowable DC current that can be drawn

from VIN.

The individual values of R3, R4 and R5 can be calculated

from the following equations:

R5 = R TOTAL •

0.8V

R ISING VIN OVLO THRESHOLD

R 4 = R TOTAL •

1.24V

R ISING VIN UVLO THRESHOLD

– R5

R3 = R TOTAL –R5 –R4

For applications that do not require an OVLO, the OVLO

pin can be tied directly to ground. The RUN pin in this type

of application can be used as an external UVLO using the

previous equations with R5 = 0Ω.

Similarly, for applications that do not require an adjustable

UVLO, the RUN pin can be tied to VIN. In this configura-

tion, the UVLO threshold is limited to the internal VIN-CAP

UVLO thresholds (VUVLO) as shown in the Electrical

Characteristics table. The resistor values for the OVLO can

be computed using the previous equations with R3 = 0Ω.

PGOOD Programming in Dropout Applications

(PGUV Pin)

The PGUV or Power Good Undervoltage pin is included

to give greater flexibility in defining a normal range of

operating VOUT for the LTC3894. In a conventional DC/DC

controller, the OV and UV comparators monitor the VFB pin

and define a fixed ± power good window about a regula-

tion point for VOUT. In the LTC3894, the OV comparator

monitors the VFB pin and the UV comparator monitors the

PGUV pin. For a typical application that does not operate

in dropout, the PGUV pin can be tied to the VFB pin to

establish a conventional ±10% PGOOD window around

the regulation point, outside of which VOUT enters UV

and OV condition respectively. Because the LTC3894 is a

100% duty cycle controller, it can be used in applications

where dropout or operating VOUT below regulation is part

of normal operation. For those applications, the PGUV

pin can be used to establish a lower limit for PGOOD

independent of the VFB pin.

A good example of defining power good in both regu-

lation and dropout is a 12V battery output preregulator

with a wide ranging VIN and VOUT regulation point set at

12V. If the defined operating output range is 12V to 9V

RUN

3894 F06

VIN

LTC3894

OVLO

Figure 6. Adjustable UV and OV Lockout

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