Rev. 1.00

6

June 26, 2015

HT7L4813

HT7L4813

Preliminary

Functional Description

The HT7L4813 is a universal AC/DC LED driver

designed for LED lighting applications. The device

can achieve high Power Factor values without

resorting to additional circuits and can also generate

high accuracy LED drive currents with very few

external components. Separate grounds are provided.

One is a floating ground for the device while the other

is the earth terminal. Users should be aware that the

two grounds cannot be directly connected together to

avoid IC damage and system malfunction.

Start-up Current

select a larger value of start-up resistor which reduces

power dissipation.

VCC Under Voltage Lockout – UVLO

The device includes a UVLO feature which has 8V

hysteresis. The PWM controller turns on when VCC

is higher than 18V and turns off when VCC is lower

than 10V. The hysteresis characteristics guarantee

that the device can be powered by an input capacitor

during start-up. When the output voltage increases to a

certain value after start-up, VCC will be charged by an

output through an auxiliary winding or a Zener Diode.

Boundary Conduction Mode – BCM

The power MOSFET is turned on by inductor current

zero-crossing detection. The current zero-crossing

can be detected by a ZCD voltage. When the inductor

current is at the zero crossing point, the voltage on the

ZCD pin will drop rapidly. The device then detects

the falling edge and turns on the internal power

MOSFET. The boundary conduction mode provides

for low turn-on switching losses and high conversion

efficiency.

Zero Current Detection – ZCD

The ZCD voltage is designed to operate between 0V

and 3V for normal operation. If the voltage on the

ZCD pin goes higher than 1.5V, the ZCD comparator

waits until the voltage goes below 1V. When the

inductor current is at the zero crossing point, the

voltage on the ZCD pin will drop rapidly. The device

will then detect the 1V falling edge and turn on the

Power MOSFET. The 0.5V hysteresis avoids any false

triggering actions due to noise.

Constant Current Control

The HT7L4813 will sense the overall inductor

current and form a closed-loop with an internal error

amplifier to obtain high constant current accuracy.

The CS voltage and the 0.2V reference voltage form

the inputs to an internal Gm amplifier whose output

is integrated via an external COMP capacitor. The

ON time of the MOSFET is controlled by the COMP

voltage to adjust the output current.

LEB on CS – Leading-Edge Blanking

Each time the internal power MOSFET is switched

on, a turn-on spike will inevitably occur at the sense

resistor. To avoid faulty triggering, a 400ns leading-

edge blank time is generated. As this function is

provided conventional RC filtering is therefore

unnecessary. During this blanking period, the current-

limit comparator is disabled and can therefore not

switch off the gate driver.

OVP on VCC – Over Voltage Protection

In order to prevent PWM controller damage, the

device includes an OVP function on VCC. Should

the VCC voltage be higher than the OVP threshold

voltage of 29V, the PWM controller will stop

operating immediately. When the VCC voltage

decreases below the UVLO off level, the controller

will reset.

LED Open Protection – ZCD OVP

The LED voltage is reflected on the ZCD pin

through a resistor RZCD. When the current on the

resistor RZCD is higher than 300μA, then ZCD OVP

protection will take place. Here the PWM controller

will stop operating immediately. When the VCC

voltage decreases below the UVLO off level, the

controller will reset.

nected between the ZCD pin and the LED positive

terminal.