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FUNCTIONAL DECRIPTIONS

The HM5668 is a high efficiency, high output voltage boost converter in a small package size. The device is ideal for driving white

LED in series. The serial LED connection provides even illumination by sourcing the same output current through all LEDs,

eliminating the need for expensive factory calibration. The device integrates 40V/0.7A switch FET and operates in pulse width

modulation (PWM) with 600kHz fixed switching frequency. For operation see the block diagram. The duty cycle of the converter

is set by the error amplifier output and the current signal applied to the PWM control comparator. The feedback loop regulates

the FB pin to a low reference voltage (200mV typical), reducing the power dissipation in the current sense resistor.

Open LED Protection

Open LED protection circuitry prevents IC damage as the result of white LED disconnection. The HM5668 monitors the voltage at

the SW pin and FB pin during each switching cycle. The circuitry turns off the switch FET and shuts down the IC when both of the

following conditions persist for 8 switching clock cycles: (1) the SW voltage exceeds the VOVP threshold and (2) the FB voltage

is less than half of regulation voltage. As a result, the output voltage falls to the level of the input supply. The device remains in

shutdown mode until it is enabled by toggling the CTRL pin logic. The maximum forward voltage plus the 200mV reference

voltage cannot exceed the minimum OVP threshold or (nLEDS X VLED(MAX)) + 200 mV ≤ VOVP(MIN).

Shutdown

The HM5668 enters shutdown mode when the CTRL voltage is logic low for more than 2.5ms. During shutdown, the input supply

current for the device is less than 1μA (max). Although the internal FET does not switch in shutdown, there is still a DC current

path between the input and the LEDs through the inductor and Schottky diode. The minimum forward voltage of the LED array

must exceed the maximum input voltage to ensure that the LEDs remain off in shutdown. However, in the typical application with

two or more LEDs, the forward voltage is large enough to reverse bias the Schottky and keep leakage current low.

Current Programming

The FB voltage is regulated by a low 0.2V reference voltage. The LED current is programmed externally using a current-sense

resistor in series with the LED string. The value of the RSET is calculated using Equation 1:

ILED =VFB/RSET

(1)

Where

ILED = output current of LEDs,VFB = regulated voltage of FB,RSET = current sense resistor

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.

PWM Dimming Control

When the CTRL pin is constantly high, the FB voltage is regulated to 200mV typically. However, the CTRL pin

allows a PWM signal to reduce this regulation voltage; therefore, it achieves LED brightness dimming. The

relationship between the duty cycle and FB voltage is given by Equation 2.

VFB =Duty*200 mV

(2)

Where

Duty = duty cycle of the PWM signal, 200 mV = internal reference voltage

This PWM dimming eliminates the audible noise which often occurs when the LED current is pulsed in replica of the frequency

and duty cycle of PWM control. Unlike other scheme which filters the PWM signal for analog dimming,HM5668 regulation voltage

is independent of the PWM logic voltage level which often has large variations For optimum performance, use the PWM dimming

frequency in the range of 5kHz to 100kHz.

HM5668