rt9903 Richtek Technology Corporation, rt9903 Datasheet - Page 13

• Manufacturer Part Number: rt9903
• Description: Channel Dc/dc Converters With High-efficiency Step-up
• Manufacturer: Richtek Technology Corporation

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-mode DC/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-mode DC/DC PWM
converter to drive external power N-MOS and external
schottky diode.
DS9903-07 August 2007
Preliminary
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
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 f
of RHPZ.
The inductor (L) and output capacitance (C
chosen first in consideration of performance, size, and
cost. The compensation resistor(R
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-(V
value, and I
target crossover frequency is 1/6 of RHPZ.
For example, if we assume V
I
then : f
f
OUT
RHPZ
(CS4)
= 0.5A, the R
=
= 10μA x R
RHPZ
V
2 L I
OUT
π
LOAD
= 66KHz
(1-D)
OUT
is the maximum output current. Typical
CS
2
LOAD
+ I
Inductor
= 6.6Ω. If we select L = 4.7uH,
IN
x R
IN
= 1.8V, V
DC(ON) External_MOS
/ V
C
) much less than that
C
OUT
) and capacitor(C
), L is the inductor
RT9903
www.richtek.com
OUT
= 3.3V, and
OUT
) are
13
C
)