ST1208
!
Function Description
The ST1208 is a complete charge-pump buck-boost converter which is requiring only four small ceramic capacitors.
In order to regulated constant current for uniform intensity, the ST1208 utilizes a proprietary 1x/1.5x fractional
charge-pump topology to drive up three white LEDs. While two digital inputs control on/off and provide brightness
control, an external resistor (R
SET
) programs the full-scale LED current. The ST1208 operates with a 1MHz fixed
frequency.
Output Regulation
The ST1208 operates in 1x mode until just above
dropout. Then the ST1208 switches to 1.5x charge
pump mode to regulate the LED1 to 300mV and
maintain constant LED brightness even at very low
battery voltages. Using this topology, there is no LED
brightness change from 1x to 1.5x. which guarantees
no flicker on the display.
The 1x mode produces almost no ripple, while the 1.5x
mode regulates the output voltage by controlling the
rate at which the charge pump capacitors are charged.
In this way, the switching frequency remains constant
for reduced input ripple and stable noise spectrum.
the IC cools by approximately 20°C.
Setting the Output Current
SET control the LED bias current. Current into
LED1, LED2 and LED3 is a multiple of the current
flowing out of SET. Set the output current as follows:
I
LEDx
= K X ( 0.6 / R
SET
)
Where K = 160, 48 or 16 ( depending upon EN1
and EN2. see Table1 ), and R
SET
is the resistor
connected between SET and GND.
Shutdown Mode
When EN1 and EN2 are grounded, the ST1208 is in
shutdown. The shutdown current is less than 1uA.
ST1208 which is out of shutdown will be operated in 1x
mode while the input voltage is higher than output
voltage.
Soft Start
The ST1208 includes soft-start circuitry to limit inrush
current at turn-on. When starting up with an output
voltage that is not near the input voltage, the output
capacitor is charged directly from the input with a
ramped current source until the output voltage is near
the input voltage. Once this occurs, the charge pump
determines if 1x or 1.5x mode is required. In the case of
1x mode, the soft start is terminated and normal
operation begins. In the case of 1.5x mode, soft-start
operates until the LED1 reaches regulation.
Thermal Shutdown
The ST1208 includes a thermal-limit circuit that
shutdown the IC at about +160°C. Turn-on occurs after
V1.1
5/12
2004/10/27
SITRONIX [ SITRONIX TECHNOLOGY CO., LTD. ]
