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LT3781

"Bootstrap” Start Dual Transistor Synchronous Forward Controller

Linear Technology 

LT3781

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 1.25V fault detector threshold on

either the OVLO or THERM pins

OVLO and THERM pins is used to directly trigger a GFC. If

either of these pins are not used, they can be disabled by

connecting the pin to ground. The intention of the OVLO

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

from an overvoltage condition though the use of a resistor

divider from the input supply. Monitoring of system tem-

perature (THERM) is possible through use of a resistor

divider using a thermistor as a 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 excessive pin input currents,

low impedance pull-up devices must not be used on these

pins.

Undervoltage Lockout

The LT3781 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 LT3781 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

LT3781 supply current to <1mA, making for efficient

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 Amplifer Configurations

The converter output voltage information is fed back to the

LT3781 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 LT3781 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 ( Figure␣ 1).

Placing a capacitor CERR from the VFB pin to the VC pin will

set the single-pole crossover frequency at (2πRFBCERR)-1.

Additional poles and zeros can be added by increasing the

complexity of the RC network.

VOUT

RFB

VFB

9

CERR

VC

10

LT3781

1.25V

3781 F01

Figure 1. Nonisolated Error Amp Configuration

3781f

11

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