LTC3803-3 Datasheet, PDF(8/16 Page) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

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LTC3803-3 Datasheet, PDF (8/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

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LTC3803-3

OPERATIO

Start-Up/Shutdown

The LTC3803-3 has two shutdown mechanisms to disable

and enable operation: an undervoltage lockout on the VCC

supply pin voltage, and a forced shutdown whenever

external circuitry drives the ITH/RUN pin low. The

LTC3803-3 transitions into and out of shutdown accord-

ing to the state diagram (Figure 1).

VCC < VTURNOFF

(NOMINALLY 5.7V)

LTC3803-3

SHUT DOWN

VITH/RUN < VITHSHDN

(NOMINALLY 0.28V)

VITH/RUN > VITHSHDN

AND VCC > VTURNON

(NOMINALLY 8.7V)

LTC3803-3 operation. The VCC voltage is then allowed to

fall to VTURNOFF (nominally 5.7V) before undervoltage

lockout disables the LTC3803-3. This wide UVLO hyster-

esis range supports the use of a bias winding on the

flyback transformer to power the LTC3803-3âsee the

section Powering the LTC3803-3.

The ITH/RUN pin can be driven below VSHDN (nominally

0.28V) to force the LTC3803-3 into shutdown. An internal

0.3Î¼A current source always tries to pull this pin towards

VCC. When the ITH/RUN pin voltage is allowed to exceed

VSHDN, and VCC exceeds VTURNON, the LTC3803-3 begins

to operate and an internal clamp immediately pulls the

ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN pin

voltage will vary from roughly 0.7V to 1.9V to represent cur-

rent comparator thresholds from zero to maximum.

LTC3803-3

ENABLED

38033 F01

Figure 1. Start-Up/Shutdown State Diagram

The undervoltage lockout (UVLO) mechanism prevents

the LTC3803-3 from trying to drive a MOSFET with insuf-

ficient VGS. The voltage at the VCC pin must exceed

VTURNON (nominally 8.7V) at least momentarily to enable

Internal Soft-Start

An internal soft-start feature is enabled whenever the

LTC3803-3 comes out of shutdown. Specifically, the

ITH/RUN voltage is clamped and is prevented from reach-

ing maximum until roughly 1.4ms has passed. This

allows the input and output currents of LTC3803-3-

based power supplies to rise in a smooth and controlled

manner on start-up.

38033fa

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