SC441D

3

Applications Information (continued)

there are some choices.

) Leave some room between Vin range and Vo.

2) Operate the Boost converter at normal load (less Ro)

3) Increases the Boost inductance (L).

Over-Current Protection

SC44D provides cycle-by-cycle current limiting for its

internal switch. If the switch current exceeds 4.A (the

typical current-limit trip point), then the current-limit

comparator ILIM, will set the latch immediately turning off

internal power. All LED current sources keep operating in

an over-current condition. Due to separate pulse-width

modulating and current limiting paths, the OCP trip point

is not affected by slope compensation (i.e. trip point is not

affected by switching duty cycle).

Over-Voltage Protection (OVP)

SC44D includes an external programming over-voltage

protection circuit to prevent damage to the IC and output

capacitor in the event of an open-circuit condition. The

boost converter’s output voltage is detected at the OVPIN

pin. If the voltage at the OVPIN pin exceeds .52V (typical),

the boost converter will shut down and a 0.9mA pull-down

currentwillbeappliedtotheVOUTpintoquicklydischarge

the output capacitor. This added protection prevents a

condition where the output capacitor and Schottky diode

must endure high voltage for an extended time, which

can pose a reliability risk for the user’s system.

Refer to evaluation application circuit in page8. The

output over voltage trip point can be programmed by R5

and R7 resistor divider.

The relationship can be described as follows:

Where OVPIN_TH is .52V typical.

An OVP event causing a fault could disable the boost

converter enabling the device to a strong pull-down. This

event would cause the FFLAG pin to go high and the soft-

start capacitor to discharge. When the soft-start capacitor

voltage falls below 0.5V, and the output voltage falls to

VIN, SC44D enters a soft-start process.

TheOVPdetectioncircuitryprovidesadisconnectfunction

during the shutdown state to prevent any leakage from

the output. The external OVP resistor divider should be

connected between VOUT and OVPRTN with the central

Referring to the Block Diagram, Figure , the clock from the

oscillation section resets the latch and turns on the power

transistor. Switch current is sensed with an integrated

sense resistor. The sensed current is summed with the

slope-compensating ramp and fed into the modulating

ramp input of the PWM comparator. The latch is set and

the power transistor conduction is terminated when

the modulating ramp intersects with the error amplifier

output (COMP).

The current-mode switching regulator is a dual-loop

feedback control system. In the inner current loop, the

EA output (COMP) controls the peak inductor current. In

the outer loop, the error amplifier regulates the output

voltage to keep the LED current at setting point. The

double reactive poles of the output LC filter are reduced

to a single real pole by the inner current loop, allowing

the simple loop compensation network to accommodate

a wide range of input and output voltages.

It is well known that, in Boost converter, Vo is greater than

or equal to Vin. In normal continuous conduction mode

(CCM) operation,

D

V

V

IN

O

−

=

1

1

Where, D is the duty ratio of the PWM power switch. As

Vin increases, in order to regulate Vo to a given constant

value, D decreases. When Vin approaches Vo, D surely

leads to 0. In practice, due to the minimum on-time of

the PWM power switch, D usually could not approach 0

with infinitely small granularity. At some point, it either

produces one pulse with minimum on-time or generates

0 by skipping the pulse. Such point could be theoretically

calculated for SC44D as follows.

For CCM: Vin ≥ 0.92 Vo.

For DCM (Discontinuous conduction mode):

Where, Ro is the Boost equivalent output resistance (=Vo/

Io), L is the Boost inductor (in uH).

In many Boost converter designs and operations, pulse

skipping is normally allowed at light load conditions.

Some designers even purposely let the Boost power con-

verter enter the pulse skipping in order to save power at

light load conditions. If some designers do not want pulse

skipping mode, based on the conditions provided above,

2

7

7

5

_

_

R

R

R

TH

OVPIN

trip

OVP

o

o

in

V

L

R

V

*

10

*

6

.

1

1

1

2

2