AME, Inc.
White LED Boost Converter
In Tiny Package
AME5142
n Application Hints
Calculating Load Current
Dimming Control
The load current is related to the average inductor cur-
rent by the relation:
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.
ILOAD = IIND (AVG) x (1 - D)
Where “D” is the duty cycle of the application. The
switch current can be found by:
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.
ISW = IIND (AVG) + 1 /2 (IRIPPLE
)
Inductor ripple current is dependent on inductance, duty
cycle, input voltage and frequency:
L1
10mH
VIN 2.7V to 5.5V
VOUT
IRIPPLE = D x (VIN-VSW) / (f x L)
CIN
4.7mF
COUT
1mF
IN
SW
Combining all terms, we can develop an expression
which allows the maximum available load current to be
calculated:
AME5142OVP
EN
Dimming
Control
25KHz to 100KHz
D ( VIN-VSW
2fL
)
I
LOAD = ( 1-D ) x ( ISW (max) -
)
FB
GND
R1
Thermal Considerations
7.5W
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:
Figure 5. PWM Dimming Control Using the EN Pin
P(SW) = D x IIND(AVE)2 x RDS(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 VIN. If the SHDN function will be needed, a
pull-up resistor must be used to VIN (approximately 50k-
100k recommended). The EN pin must not be left
unterminated.
Rev.A.01
9
AME [ ANALOG MICROELECTRONICS ]
