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■Operation

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ELM901 is designed to drive high luminosity LED which is powered by 1 lithium-ion/polymer battery with

constant current. When the battery voltage is much higher than LED forward voltage, LED is lit by connecting

directly to the battery; output voltage is stepped up to 5.5V by internal charge pump circuit to light LED when

voltage drops. LED current is driven by the constant current circuit which is set by external programming resis-

tors. With 2 types of LED current programming resistors, 3 current-driving options can be designed by different

combinations of enable circuit. ELM901A is capable of supplying output current up to 1A and thus able to pro-

vide high luminosity. To avoid LED burnout caused by longtime lighting by large current, the internal lighting

timer of ELM901A can shut down current forcibly. The timer only operates when maximum setting current is

on and can be set to 3 options-0.2s, 0.5s, 1s by external resistors connected to FS. This timer can be turned off

when it is short between FS and VIN or FS is open.

Operation right after power is supplied

ELM901 becomes standby operation when EN1=EN2=0 or open(standby); unde this operation, high

resistance PMOS switch between VIN and VO becomes ON. PMOS switch (=switch 4) between CP and

VO and NMOS switch (=switch 1) between CM and PGND1 also become on. Hence, output capacitor Co

and flying capacitor C1 are gradually charged through high resistance PMOS switch; it takes 5ms to full-

charge after the power is on. Rush current may happen when operation starts within 5ms after power is on.

Standby mode

In this mode, output capacitor Co and flying capacitor become fully charged, and through individual high

resistance PMOS switch, VIN is turned to be output mode. With maintenance of fully-charged mode during

standby situation, it is possible to avoid rush current when operation begins and lighten LED rapidly.

Start-up mode

During start-up, ELM901 detects VO short first; the low resistance switch between VIN-VO becomes on

and operates in 1x mode if short is not detected. When short is detected, Vo maintains the same condi-

tion as in standby mode, the output of LED also remains off while other internal circuits are operated nor-

mally. 1x mode is operated right after short is solved. It takes 30ms(typ.) from start-up to 1x mode output.

Vo=Vin under normal operation, and inrush current does not occur due to the charge of capacitor. When

abnormality (Vo>0.7V) such as short between VO and GND which is resulted from resistance of 10~1KΩ,

inrush current occurs due to the charge of capacitor. LED current becomes ON after 130μs(typ.) after starp-

up; it takes less than 100us since LED current becomes ON until the set current flows.

1x mode

When dropout of LED pin is detected under 1x mode, the operation switches to 2x mode automatically.

When LED current works under “low” current mode, the operation remains 1x mode for around 150ms

even if the dropout of LED pin is detected; if dropout is dissolved during this period of time, timer is reset

and 1x mode continues. When the operation is by “high” or “low+high” current, it becomes 2x mode after

2ms; if dropout is dissolved during this period of time, timer is reset and 1x mode continues. Dropout de-

tection voltage is in proportion to Iled when it is above 240mA(typ.) and the voltage is regulated to 100mV

when Iled is small.