AME, Inc.
White LED Boost Converter
In Tiny Package
AME5142
n Detailed Description
n Application Hints
The AME5142 is a constant frequency step-up con-
verter with an internal switch. The operations ofAME5142
can be understood from block diagram clearly figure.2.
The oscillator triggers the SET input of SR latch to turn
on the power switch MS at the start of each cycle. A
current sense voltage sum with a stabilizing ramp is con-
nected to the positive terminal of the PWM comparator.
When this voltage exceeds the output voltage of the error
amplifier, the SR latch is reset to turn off the power switch
till next cycle starts. The output voltage of the error am-
plifier is amplified from the difference between the refer-
ence voltage 0.15V and the feedback voltage. In this man-
ner, if the error amplifiers voltage increases, more current
is delivered to the output; if it decreases, less current is
delivered. A 26V Zener diode connects from OVP pin to
FB pin internally to provide an optional protection func-
tion which prevents SW pin from over-voltage damage.
Especially when the case of the feedback loop broken
due to component wear-out or improper connection oc-
curs. The behavior of OVP is to clamp the output voltage
to 26V typically. This function is suitable for the applica-
tions while driving white LEDs less than 6 in series.
Inductor Selection
The recommended value of inductor for AME5142 ap-
plications is 10mH. Small size and better efficiency are
the major concerns for portable device, such asAME5142
used for dual panel mobile phone. The inductor should
have low DCR for better efficiency. To avoid inductor satu-
ration, current rating should be at least 1A. The input
range is 2.7V to 5.5V.
Capacitor Selection
4.7mF input capacitor can reduce input ripple. For bet-
ter voltage stability, to increase the input capacitor value
or using LC filter is feasible, especially in the Li-ion bat-
tery application. 1mF output capacitor is sufficient to re-
duce output voltage ripple. For better voltage filtering,
ceramic capacitors with low ESR are recommended. X5R
and X7R types are suitable because of their wider volt-
age and temperature ranges.
Diode Selection
Schottky diode is a good choice for AME5142 because
of its lower forward voltage drop and faster reverse recov-
ery. Using schottky diode can get better efficiency. The
high speed rectification is also a good characteristic of
schottky diode for high switching frequency. Current rat-
ing of the diode must meet the root mean square of the
peak current and output average current multiplication.
Current Limit Protection
TheAME5142 has current limiting protection to prevent
excessive stress on itself and external components dur-
ing overload conditions. The internal current limit com-
parator will disable the NMOS power device at a typical
switch peak current limit of 850mA.
Duty Cycle
The maximum duty cycle of the switching regulator
determines the maximum boost ratio of output-to-
input voltage that the converter can attain in mode of
operation. The duty cycle for a given boost application is
defined as: This applies for continuous mode operation.
Output Over-Voltage Protection
TheAME5142 contains dedicated circuitry for monitor-
ing the output voltage. In the event that the primary LED
network is disconnected the output will increase and be
limited to 26V (TYP), which will turn the NMOS off when
the output voltage is at 26V (max.) until the output volt-
age reach 26V (TYP.) or lower. The 26V limit allows the
use of 26V 1mF ceramic output capacitors creating an
overall small solution for white LED applications.
VOUT + VDIODE - VIN
D =
VOUT + VDIODE - VSW
Under Voltage Protection
TheAME5142 has an UVP comparator to turn theNMOS
power device off in case the input voltage or battery volt-
age is too low preventing an on state of the power device
conducting large amounts of current.
Rev.A.01
8
AME [ ANALOG MICROELECTRONICS ]
