Ap5101, New prod uc t applications information, Figure 3. functional block diagram – Diodes AP5101 User Manual
Page 8
AP5101
1.5A Step-Down Converter with 1.4MHz Switching
Frequency
AP5101
Document number: DS32258 Rev. 1 - 2
8 of 15
July 2010
© Diodes Incorporated
NEW PROD
UC
T
Applications Information
Operation
The AP5101 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. 3 depicts the functional block diagram of
AP5101.
The operation of one switching 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 switching 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 (V
FB
) is lower than the reference voltage of 0.81V. This also increases the inductor current
as it is proportional to the EA voltage.
REGULATOR
REFERENCE
RAMP
GENERATOR
OSCILLATOR
1.4MHz/500KHz
RSEN
25mO
+
-
CURRENT SENSE AMPLIFIER
R
S
Q
R
DRIVER
REGULATOR
7
1
5
4
3
2
IN
EN
FB
GND
+
-
EA
ERROR AMPLIFIER
+
-
+
-
BST
SW
PWM
COMPARATOR
CURRENT LIMIT
COMPARATOR
6
COMP
8
GND
Figure 3. Functional Block Diagram