AN30888A
Product Standards
Page 18 of 21
(10) Under Voltage Lock Out (UVLO)
Under Voltage lock out prevents LSI from operation at supply voltage lower than 2.1 V.
This function prevent LSI from abnormal operation when supply voltage V
IN
drops below our recommended input
range. When input voltage is lower than this lock out value of 2.1 V, external MOSFET will be switched off. When
input voltage rises to 2.4 V or more, device operation starts again. This means a hysteresis voltage of about 0.3 V.
(11) Power MOSFET consideration
When selecting the power MOSFET, it is important to consider parameters such as gate-source, drain-source
breakdown voltage, total gate capacitance, ON resistance and the drain current rating.
When power is turned on for LSI operating in boost mode, output voltage needed to drive LED will be reflected to
Drain-Source voltage of the power MOSFET. Thus it is recommended to select a MOSFET that can handle this
output voltage. Alternatively, output and Drain-Source voltage can be protected and clamped by OVP circuit as
mentioned in point (9).
Gate capacitance of the MOSFET chosen should ideally to be smaller than 3 nF.
(12) PWM operation
PWM signal can be generated externally and input into PWM pin of this LSI. This PWM signal will turn on and off the
output driver, giving an average output LED current that is proportional to the duty cycle of the PWM signal.
I
LED(avg.)
= I
LED
Duty……………………………………… Eq[12] (Boost / Buck-Boost / Buck mode)
I
LED(avg.)
= The average output LED current after PWM is input
I
LED
= The nominal LED current set in part (7)
Duty = The ratio of on pulse time compared to total period time of the PWM signal.
A PWM frequency of 1 kHz or lower is recommended to minimize error due to rise and fall time of the converter
output.
OPERATION (continued)
Functions and properties descriptions (continued)
