RT9378 RICHTEK [Richtek Technology Corporation], RT9378 Datasheet - Page 7

• Manufacturer Part Number: RT9378
• Description: 4 CHs 100mA x1/x1.5/x2 Charge Pump White LED Driver
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Applications Information
The RT9378 uses a fractional switched capacitor charge
pump to power up to four white LEDs with a programmable
current for uniform intensity. The part integrates current
sources and automatic mode selection charge pump. It
maintains the high efficiency by utilizing an x1/x1.5/x2
fractional charge pump and current sources. The small
equivalent x1 mode open loop resistance and ultra-low
dropout voltage of current source extend the operating
time of x1 mode and optimize the efficiency in white LED
applications.
Input UVLO
The input operating voltage range of the LED driver is from
2.8V to 4.5V. An input capacitor at the VIN pin could reduce
ripple voltage. It is recommended to use a ceramic 1μF or
larger capacitance as the input capacitor. This RT9378
provides an under voltage lockout (UVLO) function to
prevent it from unstable issue when startup. The UVLO
threshold of input rising voltage is set at 2V typically with
a hysteresis of 100mV.
Soft Start
The charge pump employs a soft-start feature to limit the
inrush current. The soft-start circuit prevents the excessive
inrush current and input voltage droop. The soft-start
clamps the input current over a typical period of 50μs.
Mode Decision
The RT9378 uses a smart mode selection method to decide
the working mode for optimizing the efficiency. Mode
decision circuit senses the output and LED voltage for
up/down selection. The RT9378 automatically switches
to x1.5 or x2 mode whenever the dropout condition is
detected from the current source and returns to x1 mode
whenever the dropout condition releases.
LED connection
The RT9378 supports up to 4 white LEDs. The 4 LEDs
are connected from VIN to pin7, 8, 9, and 10 respectively.
If the LED is not used, the LED pin should be connected
to VIN directly.
DS9378-03 April 2011
PWM Dimming
2.8V to 4.5V
Capacitor Selection
To get the better performance of the RT9378, the selection
of peripherally appropriate capacitor and value is very
important. These capacitors determine some parameters
such as input/output ripple voltage, power efficiency and
maximum supply current by charge pump. To reduce the
input and output ripple effectively, the low ESR ceramic
capacitors are recommended. For LED driver applications,
the input voltage ripple is more important than output
ripple. Input ripple is controlled by input capacitor CIN,
increasing the value of input capacitance can further reduce
the ripple. Practically, the input voltage ripple depends on
the power supply impedance. The flying capacitor CFLY1
and CFLY2 determine the supply current capability of the
charge pump to influence the overall efficiency of the
system. The lower value will improve efficiency. Howere,
it will limit the LED’ s current at low input voltage. For 4
X25mA load over the entire input range of 2.8V to 4.5V, it
is recommended to use a 0.22μF ceramic capacitor on
the flying capacitor CFLY1 and CFLY2.
Brightness Control
The RT9378 implements a PWM dimming method to
control the brightness of white LEDs. When an external
PWM signal is connected to the EN pin, brightness of
white LED is adjusted by the duty cycle. The suggest
PWM dimming frequency is 1kHz to 4kHz and the PWM
minimum turn on time must be >20μs.
Figure 1. Application Circuit for One Channel Disabled
V
BAT
C
1µF
IN
GND
VIN
EN
C1P
0.22µF
C
FLY1
C1N C2P
RT9378
0.22µF
C
FLY2
VOUT
LED1
LED2
LED3
LED4
C2N
RT9378
www.richtek.com
C
1µF
OUT
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