LTC3705

10

3705fb

UVLO

The UVLO pin is connected to a resistive voltage divider

introduce hysteresis, the LTC3705 draws 4.9

µA from the

user adjustable and depends on the value of R1. The UVLO

VV

RR

R

RA

IN UVLO RISING

(,

)

(.

)

( .

)

=

+

+µ

1 242

12

2

14 9

the volt-second limit circuit. The duty cycle is always

limited to 50% to ensure that the power transformer flux

always has time to reset before the start of the next cycle.

In an alternate application, the volt-second limit can be

used for open-loop regulation of the output against changes

Current Limit

Current limit for the LTC3705 is principally a safety feature

to protect the converter and is not part of a control

function. The current that flows in series through the top

switch, the transformer primary, and the bottom switch is

sensed by a resistor connected between the source of the

bottom switch and GND. If the voltage across this resistor

exceeds 300mV, the LTC3705 initiates a fault.

Bootstrap Refresh

The LTC3705 incorporates a unique bootstrap refresh

circuit to ensure that the bootstrap supply (BOOST) for the

top switch has adequate voltage for operation at low duty

cycles. Therefore, the LTC3705 does not require a

undervoltage lockout for the bootstrap supply and a po-

tential source of unexpected shutdowns is eliminated.

Voltage Feedforward

The LTC3705 uses voltage feedforward to properly modu-

late the duty cycle as a function of the input voltage. For

secondary-side control with the LTC3706, voltage

feedforward is used during start-up only. The duty cycle

during start up is determined by comparison of the voltage

on the SSFLT pin to a 50% duty cycle triangle wave with

an amplitude of 2V. To implement voltage feedforward, the

charging current for the soft-start capacitor is reduced in

proportion to the input voltage. As a result, the initial rate

of rise of the converter output voltage is held approxi-

mately constant regardless of the input voltage. At some

point during start-up, the LTC3706 begins to switch the

pulse transformer and takes over the soft-start.

For operation with standalone primary-side control and

optoisolator feedback, voltage feedforward is used during

both start-up and normal operation. The duty cycle is

determined by using a 50% duty cycle triangle wave with

an amplitude equal to 66% of the voltage on the UVLO pin

for the soft-start capacitor is a constant 5.2

µA. During

soft-start, the duty cycle is determined by comparing the

voltage on the SSFLT pin to the triangle wave. Soft-start is

concluded when the voltage on the SSFLT pin exceeds the

start, the duty cycle is determined by comparison of the

Optoisolator Bias

When the LTC3705 is used in standalone primary-side

mode, feedback is provided by an optoisolator connected

bias circuit which eliminates the need for external

components.

OPERATIO

APPLICATIO S I FOR ATIO

The LTC3705 also has 16mV of voltage hysteresis on the

VV

RR

R

IN UVLO FALLING

(,

)

(.

)

=

+

1 226

12

2

To implement external Run/Stop control, connect a small

NMOS to the UVLO pin as shown in Figure 2. Turning the

NMOS on grounds the UVLO pin and prevents the LTC3705

from running.