AME
1.5MHz, 600mA
Synchronous Buck Converter
AME5248A
nDetailed Description
Main Control Loop
Current Limit Protection
The AME5248A utilizes a fixed-frequency,current-mode
PWM control scheme combined with fully-integrated
power MOSFETs to produce a compact and efficient step-
down DC-DC solution. During normal operation the high-
side MOSFET turns on each cycle and remains on until
the current comparator turns it off. At this point the low-
side MOSFET turns on and remains on until either the
end of the switching cycle or until the inductor current
approaches zero. The error amplifier adjusts the current
comparator's threshold according to the load current to
ensure that the output voltage remains in regulation.
The AME5248A has current limiting protection to pre-
vent excessive stress on itself and external components.
The internal current limit comparator will disable the
power device at a switch peak current limit. Under ex-
treme overloads, such as short-circuit conditions, the
AME5248A reduces it's oscillator frequency to around
210KHz to allow further inductor current reduction and to
minimize power dissipation.
Under Voltage Protection
Light Load Power Saving Mode Operation
The AME5248A has an UVP comparator to turn the power
device off in case the input voltage or battery voltage is
too low.
The AME5248A is capable of Power Saving Mode Op-
eration in which the internal power MOSFETs operate
intermittently based on load demand.
In Power Saving Mode operation, the peak current of
the inductor is set to a certain value which increases as
the input voltage increases, such as 260mA for 3.6V in-
put voltage and 340mA for 5.5V input voltage, approxi-
mately. Each switching event can last from a single cycle
at very light loads to few cycles within the active intervals
at moderate loads. Between these switching intervals,
the unneeded circuitry are turned off, reducing the quies-
cent current to 20mA. In this turned off state, the load
current is being supplied solely from the output capaci-
tor. As the output voltage droops, the internal comparator
trips and turns on the circuits. This process repeats at a
rate depends on the load demand.
Soft Start
The AME5248A integrates a soft start function that pre-
vents input inrush current and output overshoot during
start-up. During start-up the switch current limit is in-
creased in steps. The start-up time thereby depends on
the output capacitor and load current demanded at start-
up. Typical start-up times with a 10mF output capacitor,
3.6V input voltage and 1.5V output voltage, for 600mA
load is 700ms, and 150ms for 1mA load.
Thermal Shutdown
The device protects itself from overheating with an in-
ternal thermal shutdown circuit. If the junction tempera-
ture exceeds the thermal shutdown trip point, the device
turns off. The part is restarted when the junction tem-
peraturedrops30oC below the thermal shutdown trip point.
Dropout Operation
As the input supply voltage decreases to a value ap-
proaching the output voltage, the duty cycle increases
toward the maximum on-time. Further reduction of the
supply voltage forces the main switch to remain on for
more than one cycle until it reaches 100% duty cycle.
The output voltage will then be determined by the input
voltage minus the voltage drop across the P-channel
MOSFET and the inductor.
Rev.A.01
9
AME [ ANALOG MICROELECTRONICS ]
