RT9903 datasheet(13/15 Pages) RICHTEK

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RT9903

Preliminary

DS9903-07 August 2007

www.richtek.com

Protection

Current Limit

The current of NMOS is sensed cycle by cycle to prevent

over current. When over current limit, then the NMOS is

off. This state is latched and then reset automatically at

next clock cycle.

Over Voltage

The over voltage protection prevents LX2 voltage going

too high. The over-voltage is detected by the junction

leakage and the threshold value is around 22V. This

channel is not latched shut down when OVP occurs.

Step-up (Boost) DC/DC Converter (CH3)

The channel (CH3) is a step-up current-modeDC/DC PWM

converter with built-in internal power MOS and external

schottky diode. This channel is designed to light 2 to 4

WLEDSs with constant current regulation. The lightness

of WLED can be dimming-controlled by the duty of EN3

pin.

The maximum duty of the constant frequency is 96% for

this channel to prevent high input current drawn from

input.

Protection

Current Limit

The current of NMOS is sensed cycle by cycle to prevent

over current. When over current limit, then the NMOS is

off. This state is latched and then reset automatically at

next clock cycle.

Over Voltage

The over voltage protection prevents LX3 voltage going

too high. The over-voltage is detected by the junction

leakage and the threshold value is around 22V. This

channel is latched shut down when OVP occurs, and can

be reset by toggling EN3.

Step-up (Boost) DC/DC Converter (CH4)

The channel (CH4) is a step-up current-modeDC/DC PWM

converter to drive external power N-MOS and external

schottky diode.

At light load, efficiency is enhanced by pulse-skipping

mode. In this mode, the external NMOS turns on by a

constant pulse width. As loading increased, the converter

operates at constant frequency PWM mode. The

maximum duty of the constant frequency is 80% for the

boost to prevent high input current drawn from input.

Protection

Current Limit

The current of NMOS is sensed cycle by cycle to prevent

over current. The current is sensed by CS4 pin to

determine whether it reaches current limit threshold.

When CS4 voltage is higher than 0.4V, the external

NMOS is off. This state is latched and then reset

automatically at next clock cycle.

V(CS4) = 10

μA x RCS + IInductor x RDC(ON) External_MOS

Current Mode Step-up Compensation

When the step-up converter operates with continuous

inductor current, the right-half-plane zero (RHPZ) appears

in the loop-gain frequency response. To ensure the

stability, the control-loop gain should crossover at the

frequency (crossover frequency fC) much less than that

of RHPZ.

The inductor (L) and output capacitance (COUT) are

chosen first in consideration of performance, size, and

cost. The compensation resistor(RC) and capacitor(CC)

are then chosen to optimize the control-loop stability.

The useful steps are listed below to calculate loop

compensation.

Step-1

Calculate RHPZ

For continuous conduction, the RHPZ is given by

Where D is the duty cycle = 1-(VIN / VOUT), L is the inductor

value, and I

LOAD is the maximum output current. Typical

target crossover frequency is 1/6 of RHPZ.

For example, if we assume VIN = 1.8V, VOUT = 3.3V, and

IOUT = 0.5A, the RLOAD = 6.6

Ω. If we select L = 4.7uH,

then : fRHPZ = 66KHz

OUT

RHPZ

OUT

(1-D)

2 L I

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