LD7890GS datasheet(11/16 Pages) LEADTREND

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LD7890

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7890-DS-01 November, 2013

blanking time to suppress the switching noise from the

external MOSFET. A PWM comparator compares the

current-sense voltage and the slop compensation signal

with the output of the GM error amplifier. The device will

turn off the external MOSFET when the voltage across CS

exceeds the error amplifier’s output voltage. This process

repeats

every

switching

cycle

achieve

peak

current-mode control.

The LD7890 is integrated with under-voltage lockout

(UVLO) function. The device will be disabled as VIN pin

’s

voltage drops below UVLO threshold of 7.5V and will not

resume until it rises up to 8.9V.

Start Up Sequence

As EN is activated, the IC will start to check if OVP

’s

voltage is above 0.2V. If confirmed, the DIM pin will signal

high to boost internal soft start. The Soft start function

would limit inductor current to let it rise linearly and

minimize the input inrush current.

Multi-Chip Parallel Operation

If used in some applications requiring more than 4 strings

or LED, just connect desired numbers of LD7890 in

parallel in a single power stage for it.

In multiple chips operation, each PARL pins should be

connected together and set the master chip to control the

driving signal to the external NMOSFET. Leave the

remaining chips work in slave mode according to the

following terms

(1): Float DRV pin

(2): Short CS to AGND

(3): Slave OVP pin Setting lower than Master

(4): LOSC pin parallel.

Loop compensation

The LD7890 has an internal error amplifier for LED

current regulation for COMP output to compensate the

control loop. The selection of the inductor, output

capacitor, compensation resistor and capacitor would

affect the loop stability. The inductor and output capacitor

are determined based on the terms of performance, size

and cost, while the compensation resistor and capacitor

for COMP are according to the criteria to optimize control

loop stability.

Switching Frequency Selection

LD7890 can operate in fixed frequency mode. The

constant operation frequency is set by an external resistor

connected between RT pin and ground. The resistor sets

the charging current for internal oscillator.

)]

(

[

70750

)

kHz

(

FSW







Programming the LED Current

The SENSE pins detect the voltage of the external LED

current bias resistors and individually regulate accurate

voltage to 0.4V. They cooperate with the internal

compensation circuits to eliminate the operating GATE

[1:4] current for NMOS

’s use to secure extremely accurate

LED current matching with each the individual and

independent channel.

)

(

)

(

SENSE

LED





External PWM dimming

PWM dimming control will be achieved by applying an

external PWM signal between 100Hz and 800Hz to the

DIM pin.

Internal PWM dimming

Apply DC Voltage (0.2~1.2V) to the DIM Pin in compare

with the LOSC signal to enable PWM Dimming. The

internal frequency is determined by the external capacitor.

If the external capacitor is 2.2nF, its internal frequency is

1KHz typically.

The PWM dimming duty is determined by the DC voltage

ranging between 0.2V and 1.2V. Larger DC voltage

usually produces greater brightness.

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