AME
AME5144
n
Detailed Description
The high efficiency and small size of the AME5144 make
them ideally suited to driver up to ten series-connected
LEDs. They operate as a boost DC-DC converter that con-
trols output current rather than voltage. A low 300mV cur-
rent-sense threshold minimized the losses. The AME5144
provide even illumination by sourcing the same output cur-
rent through each LED, eliminating the need for expensive
factory calibration. The fast 1MHz internal oscillator al-
lows for small inductor and small input and output capaci-
tors while minimizing input and output ripple.
The single analog control input (CTRL) allows easy ad-
justment of LED brightness and on/off control. This allows
simple logic-level on/off control, analog voltage control, or
PWM duty-cycle control of both brightness and shutdown.
In shutdown, supply current is reduced to a low 0.3µA
(Typ.). A soft-start gradually illuminates the LEDs, elimi-
nating the inrush current during startup.
High Efficiency, 37V Step-Up
Converter for 2 to 10 White LEDs
Over Voltage Protection
OVP is designed to prevent the output voltage from ex-
ceeding the maximum switch voltage rating of 38.5V (Typ.).
The protection circuitry stops the internal MOSFET from
switching. There is a 1.5V hysteresis associated with
this circuitry which will cause the output to fluctuate be-
tween 38.5V and 37V.
Adjusting LED Current
Adjusting the output current of the AME5144 changes
the brightness of the LEDs. An analog input (CTRL) and
the sense-resistor values set the output current. Output
current is given by:
I
LED
=V
CTRL
/(5xR
SENSE
)
The V
CTRL
voltage range for adjusting output current is
0.24V to 1.65V. To set maximum current, calculate
RSENSE when VCTRL is at its maximum as follows:
R
SENSE
=1.65V / (5 x I
LED
)
Power dissipation in R
SENSE
is typical less than 10mW,
allowing the use of small surface-mount resistor.
Shutdown
The AME5144 enter shutdown when VCTRL is less than
100mV for more than 8.2ms. When shutdown, the supply
current is reduced to 0.3µA (Typ.) approximately.
The 0.3uA supply current is applied for the voltage-de-
tection circuitry. C
COMP
is discharge during shutdown, al-
lowing the device to reinitiate soft-start when enabled. Al-
though the internal n-channel MOSFET does not switch in
shutdown, there is still a DC current path between the
input and the LEDs through the inductor and Schottky di-
ode. The minimum forward voltage of the LED array must
exceed the maximum input voltage to ensure that the LEDs
remain off in shutdown. However, with two or more LEDs,
the forward voltage is enough to keep leakage current low,
less than 1µA (Typ.).
PWM Dimming Control
A. Using DC Signal for CTRL Pin
For CTRL, applying a DC signal in the range of 0.24V to
1.65V control the LED current. CTRL can be overdriven;
however, applying a CTRL greater than 1.65V does not
increased the LED current above the level at 1.65V.
B. Using PWM signal for CTRL pin
c
The CTRL input is used as a digital input allowing LED
brightness control with a logic-level PWM signal applied
directly to CTRL. The frequency range is from 200Hz to
30KHz, while 0% duty cycle corresponds to minimum cur-
rent, and 100% duty cycle corresponds to full current.
The error amplifier and compensation capacitor form a
low pass filter so PWM dimming results in DC current to
the LEDs without the need for any additional RC filters.
Soft-Start
The AME5144 attain soft-start by charging C
COMP
gradu-
ally with a current source. When V
COMP
rises above 1.25V,
the internal MOSFET begins switching at a reduced duty
cycle. When V
COMP
rises above 2.25V, the duty cycle is
at its maximum. See the Typical Operation Characteris-
tics for an example of soft-start operation.
8
Rev. D.01
AMETHERM [ AMETHERM CIRCUIT PROTECTION THERMISTORS ]
