AME5142 Datasheet, PDF (7/18 Pages) Analog Microelectronics – White LED Boost Converter In Tiny Package

AME, Inc.
AME5142
White LED Boost Converter
In Tiny Package
n Application Hints
Calculating Load Current
The load current is related to the average inductor cur-
rent by the relation:
I_LOAD = I_IND (AVG) x (1 - D)
Where “D” is the duty cycle of the application. The
switch current can be found by:
I_SW = I_IND (AVG) + 1 /2 (I_RIPPLE)
Inductor ripple current is dependent on inductance, duty
cycle, input voltage and frequency:
I_RIPPLE = D x (V_IN-V_SW) / (f x L)
Combining all terms, we can develop an expression
which allows the maximum available load current to be
calculated:
I_LOAD
( 1-D ) x ( I_SW (max) - ^{D ( VIN-VSW )} )
2fL
Thermal Considerations
At higher duty cycles, the increased ON time of the
FET means the maximum output current will be deter-
mined by power dissipation within the AME5142 switch.
The switch power dissipation from ON-state conduction
is calculated by:
Dimming Control
A. Using a PWM Signal to EN Pin
For controlling the LED brightness, the AME5142 can
perform the dimming control by applying a PWM signal to
EN pin.
The average LED current is proportional to the PWM
signal duty cycle. The magnitude of the PWM signal should
be higher than the maximum enable voltage of EN pin, in
order to let the dimming control perform correctly.
V_IN 2.7V to 5.5V
L1
10µH
C_IN
4.7µF
IN
SW
V_OUT
C_OUT
1µF
Dimming
EN ^AME5142 _OVP
Control
25KHz to 100KHz
GND
FB
R1
7.5Ω
Figure 5. PWM Dimming Control Using the EN Pin
P_(SW) = D x I_IND(AVE)2 x R_DS(ON)
There will be some switching losses as well, so some
derating needs to be applied when calculating IC power
dissipation.
Shutdown Pin Operation
The device is turned off by pulling the shutdown pin low.
If this function is not going to be used, the pin should be
tied directly to V . If the SHDN function will be needed, a
IN
pull-up resistor must be used to V_IN (approximately 50k-
100k recommended). The EN pin must not be left
unterminated.
Rev. A.01
9