RT9903 Datasheet, PDF (14/15 Pages) Richtek Technology Corporation – 5 Channel DC/DC Converters IC with High-Efficiency Step-Up

RT9903
Preliminary
Step-2 Calculate C_C
Choose f_C = 10KHz, and then C_C is calculated from the
simplified loop-gain formula.
Loop gain =
Gm x ^{1 + SRCCC} x ¹ x (1-D) x
ROUT
x ^VFB
SCC
RCS
1+SROUTCOUT VOUT
Where Gm is the transconductance of error amplifier,
and R_CS is the current sense amplifier transresistance.
In our design, Gm=200us, R_CS = 0.8V/A, and V_FB = 1V,
and then C_C = 4.34nF from calculation.
Choose C_C = 4.7nF.
Step-3 Calculate R_C
R_C is calculated such that transient droop requirements
are met.
For example, in our design, if 5% transient droop is
allowed, then the error amplifier moves 0.05 x 1V, or
50mV. The error amplifier output drives 50mV x 200us,
or 10uA across R_C to provide transient gain.
We select R_CS = 2.7kΩ to meet the requirements.
The output capacitor is chosed 40uF to cancel the R_C
C_C zero, and can sustain stable V_OUT voltage at heavy
load condition.
Charge Pump DC/DC Converter (CH5)
This is a low quiescent negative-voltage charge pump
DC/DC converter, which is enabled by EN2. Output ripple
can be easily suppressed by increasing the capacitance
ratio of C_OUT and C_CP. This charge pump DC/DC converter
can apply to negative voltage of CCD.
The maximum output current is determined by the ratio
of C_CP and C_OUT. This equation would describe the
relationship.
I_MAX = (V_INCD − 2V_F) x C_CP x F_CP
z V_F : Schottky diode forward voltage
z F_CP : Charge pump maximum frequency is 500kHz.
The negative output voltage is set by R1 and R2. The
FBCP threshold voltage is 0V.
VOUT x R2
=1
VDD x R1
INCD
Charge-
Pump
C_X
C_CP
Driver
FBCP
+
-
V_DD
Negative
Output
Voltage
R1
C_OUT
R2
Reference
The chip has an internal 1V reference voltage, which is
the inputs of the error amplifiers of the CH1, CH2, and
CH4 to compare the difference of feedback voltage. The
reference voltage of CH3 is 0.2V for WLEDs application.
The reference voltage can be set up stably when the
supplied power (VDD) is above 1.5V, and EN1 (or EN2,
EN3, EN4 either one) goes high.
Thermal Protection
Thermal protection function is integrated in the chip. When
the chip temperature is higher than 180°C, the controllers
of all channels are shutdown. When the thermal protection
is relieved, the chip operates well again.
www.richtek.com
14
DS9903-07 August 2007