Function Description

Main Control Loop

The FTD8203 is a constant frequency current-mode switching

regulator. During normal operation, the internal N-channel power

MOSFET is turned on each cycle when the oscillator sets an

internal RS latch and turned off when an internal comparator

(ICMP) resets the latch. The peak inductor current at which ICMP

resets the RS latch is controlled by the voltage on the COMP

node, which is the output of the error amplifier (EAMP). An

external current-sense resistor connected between cathode of

the lowest LED and ground allows the EAMP to receive a current

decreases to cause the LEDs current to decrease,it causes a

turn causes the COMP voltage to increase until the LEDs current

reaches the set point.

VIN Under-Voltage Lockout (UVLO)

The Under-Voltage Lockout (UVLO) circuit compares the input

voltage at VIN with the UVLO threshold (2.2V rising,typical) to

ensure the input voltage is high enough for reliable operation.

The 100mV (typ) hysteresis prevents supply transients from

causing a restart. Once the input voltage exceeds the UVLO

rising threshold, startup begins.When the input voltage falls

below the UVLO falling threshold, the controller turns off the

converter.

Soft-Start

The FTD8203 has a built-in soft-start to control the N channel

MOSFET current raises during start-up. During soft-start, an

internal ramp voltage connected to one of the inverting inputs of

the current limit comparator. The inductor current limit is

proportional to the voltage. When the threshold voltage of the

internal soft-start comparator is reached, the full current limit is

released.

Current-Limit Protection

The FTD8203 monitors the inductor current, flowing through the

N-channel MOSFET, and limits the current peak at current-limit

level to prevent loads and the device from damages in overload

conditions.

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction temperature of the

FTD8203.When the junction temperature exceeds150℃, a thermal

sensor turns off the power MOSFET, allowing the device to cool.

The thermal sensor allows the converter to start a soft-start

process and regulate the LEDs current again after the junction

temperature cools by 40℃. The OTP is designed with a 40℃

continuous thermal overload conditions, increasing the lifetime of

the device.

Enable/Shutdown

Driving EN to ground places the FTD8203 in shutdown mode.When

in shutdown, the internal power MOSFET turns off, all internal

circuitry shuts down and the quiescent supply current reduces to

1uA maximum. This pin also could be used as a digital input

allowing brightness controlled by using a PWM signal with

frequency from 100Hz to 100kHz. The 0% duty cycle of PWM

signal corresponds to zero LEDs current and 100% corresponds to

full one. Suggestion dimmimg duty range is from 8% to 100% at

100kHz dimmimg frequency.

Open-LED Protection

In driving LED applications, the feedback voltage on FB pin falls

down if one of the LEDs, in series, is failed.Meanwhile, the

converter unceasingly boosts the output voltage like an open-loop

operation. Therefore, an overvoltage protection monitoring the

output voltage via OVP pin is integrated into the chip to prevent the

LX and the output voltages from exceeding their maximum voltage

ratings. Once the voltage on the OVP pin rises above the OVP

threshold, the converter stops switching and prevents the output

voltage from rising. The converter can work again when the OVP

voltage falls below the falling of OVP voltage threshold.

White LED Step-up Converter

2019. 02. 08

10/13

Revision No : 0

FTD8203