BAIDU MICRO ELECTRONS CO., LTD.
1.2A, 24V, 1.4MHz Step-down Converter
APPLICATION INFORMATION
OPERATION
The AE2359 is a current mode control, asynchronous buck regulator. Current mode control assures
excellent line and load regulation and a wide loop bandwidth for fast response to load transients.
Figure. 4 depicts the functional block diagram of AE2359.
The operation of one switching cycle can be explained as follows. At the beginning of each cycle, HS
(high-side) MOSFET is off. The EA output voltage is higher than the current sense amplifier output,
and the current comparator’s output is low. The rising edge of the 1.4MHz oscillator clock signal sets
the RS Flip-Flop. Its output turns on HS MOSFET.
When the HS MOSFET is on, inductor current starts to increase. The Current Sense Amplifier
senses and amplifies the inductor current. Since the current mode control is subject to sub-harmonic
oscillations that peak at half the switching frequency, Ramp slope compensation is utilized. This will
help to stabilize the power supply. This Ramp compensation is summed to the Current Sense
Amplifier output and compared to the Error Amplifier output by the PWM Comparator. When the sum
of the Current Sense Amplifier output and the Slope Compensation signal exceeds the EA output
voltage, the RS Flip-Flop is reset and HS MOSFET is turned off. The external Schottky rectifier diode
(D1) conducts the inductor current.
For one whole cycle, if the sum of the Current Sense Amplifier output and the Slope Compensation
signal does not exceed the EA output, then the falling edge of the oscillator clock resets the Flip-Flop.
The output of the Error Amplifier increases when feedback voltage (VFB) is lower than the reference
voltage of 0.81V. This also increases the inductor current as it is proportional to the EA voltage.
Setting the Output Voltage
The output voltage can be adjusted from 0.81V to 15V using an external resistor divider. Table 1
shows a list of resistor selection for common output voltages. Resistor R1 is selected based on a
design tradeoff between efficiency and output voltage accuracy. For high values of R1 there is less
current consumption in the feedback network. However the trade off is output voltage accuracy due
to the bias current in the error amplifier. R2 can be determined by the following equation:
AE2359
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BAIDUMICRO [ BaiDu Micro electrons Co., Ltd ]
