SC5010 datasheet(16/36 Pages) SEMTECH

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SC5010

Applications Information (continued)

Boost Converter Operation

The SC5010 includes a boost controller with programma-

ble switching frequency. It applies current-mode control

method with integrated compensation loop. The clock

(see block diagram) from the oscillator sets the latch and

turns on the external power MOSFET, which serves as the

main power switch. The current flowing through this

switch is sensed by the current sense resistor in series

with the switch. The sensed switch current is summed

with the slope-compensated ramp and fed into the mod-

ulating input of the PWM comparator. When the modulat-

ing ramp intersects the error amplifier output (COMP),

the latch is reset and the power MOSFET is turned off. The

sense resistor also sets the peak current limit of the power

MOSFET, I

OCP

using the following equation:

OCP[A] = 0.4 / RCS [Ω]

The current-mode control system contains two loops. For

the inner current loop, the Error Amplifier (EA) output

(COMP) controls the peak inductor current. In the outer

loop, the EA regulates the output voltage for driving the

LED strings.

Boost Converter Switching Frequency Selection

The resistor between FSET and GND sets the boost con-

verter switching frequency (200kHz to 2.2MHz) using the

following equation:

[kHz] = 105/ R

FSET

[kΩ]

Higher switching frequency allows the use of low profile

height inductor for space-constrained and cost-sensitive

applications.

Over-Voltage Protection (OVP)

SC5010 features programmable output over-voltage pro-

tection preventing damage to the IC and output capacitor

in the event of LED string open-circuit. The boost converter

output voltage is sensed at the OVP pin through resistor

voltage divider. The OVP trip threshold (refer to detailed

application circuit on page 21) can be calculated using the

following equation.

Output OVP Trip Voltage [V] = 1.2 X (R

+ R

) / R

When the OVP pin voltage exceeds 1.2V, the boost con-

verter turns off and the FLT pin is pulled low. When the

OVP pin voltage falls below the OVP threshold (falling),

the boost converter restarts and the FLT pin is released.

There is 10mV hysteresis between OVP pin threshold

(falling) and OVP pin threshold (rising). This results in an

output voltage hysteresis given by:

Output OVP Hysteresis[mV] = 10 X (R

+ R

) / R

LED Current Sink

The SC5010 provides 8 current sinks and each can sink up

to 30mA current. It incorporates LED string short-circuit

protection (trip level programmable and can be disabled

as well), LED string open-circuit protection.

LED String Peak Current Programming

LED string peak current (at 100% dimming) can be set by

selecting resistor R

ISET

, connected between ISET and GND.

The relationship between R

ISET

resistance and single LED

string peak current is calculated using the following

equation:

LED

[mA] = (1055 X 1.23) / R

ISET

[kΩ]

The LED string current can be programmed up to 30mA.

Unused Strings

The SC5010 may be operated with less than 8 strings. In

this mode of operation, all unused IO pins should be con-

nected to ground. During startup, these unused strings are

detected and disabled while other active strings work

normally.

LED Short-Circuit Protection (SCP)

SC5010 features a programmable LED short protection.

This allows the part to be customized based on the LED V

mismatches between the LED strings. If one or more LEDs

are detected as short-circuited, the corresponding string

will be latched off. The voltages on all IO pins are monitored

to check if any IO pin exceeds the SCP trip point (The IO

voltage for LED string with faulty short-circuit LED(s) will be

higher than other normal IO pin voltages). This LED Short-

Circuit Protection (SCP) trip level (see detailed application

circuit on page 21) is given by the following equation.

SCP_Trip [V] = 20 X (1.23 X R4) / (R4 + R5)

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