AME, Inc.
1.6 MHz Boost Converter with
30V Internal FET Switch
AME5140
n Application Hints
n Detailed Description
The AME5140 is a switching converter IC that operates
at a fixed frequency (1.6MHz) for fast transient response
over a wide input voltage range and incorporates pulse-by-
pulse current limiting protection. Operation can be best
understood by referring to Figure 4. Because this is cur-
rent mode control, a 33mW sense resistor in series with
the switch FET is used to provide a voltage (which is pro-
portional to the FET current) to both the input of the pulse
width modulation (PWM) comparator and the current limit
amplifier.
Selecting The External Capacitors
The best capacitors for use with the AME5140 are
multilayer Ceramic capacitors. They have the lowest
ESR (equivalent series resistance) and highest resonance
frequency, which makes them optimum for use with high
frequency switching Converters. When selecting a ce-
ramic capacitor, only X5R and X7R dielectric types should
be used. Other types such as Z5U and Y5F have such
severe loss of capacitance due to effects of temperature
variation and applied voltage, they may provide as little
as 20% of rated capacitance in many typical applica-
tions. Always consult capacitor manufacturer’ s data
curves before selecting a capacitor. High-quality ceramic
capacitors can be obtained from Taiyo-Yuden,AVX, and
Murata.
At the beginning of each cycle, the S-R latch turns on the
FET. As the current through the FET increases, a voltage
(proportional to this current) is summed with the ramp com-
ing from the ramp generator and then fed into the input of
the PWM comparator. When this voltage exceeds the volt-
age on the other input (coming from the Gm amplifier), the
latch resets and turns the FET off. Since the signal coming
from the Gm amplifier is derived from the feedback (which
samples the voltage at the output), the action of the PWM
comparator constantly sets the correct peak current through
the FET to keep the output voltage in regulation.
Selecting The Output Capacitor
A single ceramic capacitor of value 4.7mF to 10mF will
provide sufficient output capacitance for most applica-
tions. If larger amounts of capacitance are desired for
improved line support and transient response, tantalum
capacitors can be used. Aluminum electrolytic with ul-
tra low ESR such as Sanyo Oscon can be used, but are
usually prohibitively expensive. Typical AI electrolytic
capacitors are not suitable for switching frequencies above
500kHz due to significant ringing and temperature rise
due to self-heating from ripple current. An output ca-
pacitor with excessive ESR can also reduce phase mar-
gin and cause instability. In general, if electrolytic are
used, it is recommended that. They be paralleled with
ceramic capacitors to reduce ringing, switching losses,
and output voltage ripple.
Q1 and Q2 align with R3 - R6 form a bandgap voltage
reference used by the IC to hold the output in regulation.
The currents flowing through Q1 and Q2 will be equal, and
the feedback loop will adjust the regulated output to main-
tain this. Because of this, the regulated output is always
maintained at a voltage level equal to the voltage at the FB
node "multiplied up" by the ratio of the output resistive di-
vider.
The current limit comparator feeds directly into the flip-
flop that drives the switch FET. If the FET current reaches
the limit threshold, the FET is turned off and the cycle ter-
minated until the next clock pulse. The current limit input
terminates the pulse regardless of the status of the output
of the PWM comparator.
Selecting The Input Capacitor
An input capacitor is required to serve as an energy
reservoir for the current which must flow into the coil
each time the switch turns ON. This capacitor must
have extremely low ESR, so ceramic is the best choice.
We recommend a nominal value of 4.7mF, but larger val-
ues can be used. Since this capacitor reduces the
amount of voltage ripple seen at the input pin, it also
reduces the amount of EMI passed back along that line
to other circuitry.
Rev.G.01
10
AME [ ANALOG MICROELECTRONICS ]
