LT1681 datasheet(11/20 Pages) LINER

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LT1681

1681f

APPLICATIO S I FOR ATIO

Events that trigger a GFC are:

a) Exceeding the current limit of the 5VREF pin

b) Detecting an undervoltage condition on VCC

c) Detecting an undervoltage condition on 5VREF

d) Pulling the SHDN pin below the shutdown threshold

e) Exceeding the IMAX pin threshold

f) Exceeding the 1.25V fault detector threshold on either

the OVLO or THERM pins

The OVLO and THERM pins are used to directly trigger a

GFC. If either of these pins are not used, they can be

disabled by connecting the pin to SGND. The intention of

the OLVO pin is to allow monitoring of the input supply to

protect from an overvoltage condition. Monitoring of

system temperature (THERM) is possible through use of

a resistor divider using a thermistor as a resistor divider

component. The 5VREF pin can provide the precision

supply required for these applications. When these fault

detection circuits are disabled during shutdown or VCC pin

UVLO conditions, a reduction in OVLO and THERM pin

input impedance to ground will occur. To prevent exces-

sive pin input currents, low impedance pull-up devices

must not be used on these pins.

Undervoltage Lockout

The LT1681 maintains a low current operational mode

when an undervoltage condition is detected on the VCC

supply pin, or when VCC is below the undervoltage lockout

(UVLO) threshold. During a UVLO condition on the VCC

pin, the LT1681 disables all internal functions with the

exception of the shutdown and UVLO circuitry. The exter-

nal 5VREF supply is also disabled during this condition.

Disabling of all switching control circuity reduces the

LT1681 supply current to < 1mA, simplifying integration

of trickle charging in systems that employ output feedback

supply generation.

The function of the high side switch output (TG) is also

gated by UVLO circuitry monitoring the bootstrap supply

(VBST-BSTREF). Switching of the TG pin is disabled until

the voltage across the bootstrap supply is greater than

7.4V. This helps prevent the possibility of forcing the high

side switch into a linear operational region, potentially

causing excessive power dissipation due to inadequate

gate drive during start-up.

Error Amplifier Configurations

The converter output voltage information is fed back to the

LT1681 onto the VFB pin where it is transformed into an

output current control voltage by the error amplifier. The

error amplifier is generally configured as an integrator and

is used to create the dominant pole for the main converter

feedback loop. The LT1681 error amplifier is a true high

gain voltage amplifier. The amplifier noninverting input is

internally referenced to 1.25V; the inverting input is the

VFB pin and the output is the VC pin. Because both low

frequency gain and integrator frequency characteristics

can be controlled with external components, this amplifier

allows far greater flexibility and precision compared with

use of a transconductance error amplifier.

In a nonisolated converter configuration where a resistor

divider is used to program the desired output voltage, the

error amplifier can be configured as a simple active

integrator, forming the system dominant pole (see Fig-

ure 1). Placing a capacitor CERR from the VFB pin to the VC

pin will set the single-pole crossover frequency at

πRFBCERR)–1. Additional poles and zeros can be added

by increasing the complexity of the RC network.

VFB

RFB

CERR

VOUT

1.25V

1681 F01

LT1681

Figure 1. Nonisolated Error Amp Configuration

Another common error amplifier configuration is for

optocoupler use in fully isolated converters with second-

ary-side control (see Figure 2). In such a system, the

dominant pole for the feedback loop is created at the sec-

ondary-side controller, so the error amplifier needs only to

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