AP6507

500 kHz 18V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6507

Document number: DS33435 Rev. 2 - 2

9 of 13

www.diodes.com

May 2011

© Diodes Incorporated

Application Information

Theory of Operation

The AP6507 is a 3A current mode control, synchronous

buck regulator with built in power MOSFETs. Current

mode control assures excellent line and load regulation

and a wide loop bandwidth for fast response to load

transients.

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 (error amplifier) output voltage is

higher than the current sense amplifier output, and the

current comparator’s output is low. The rising edge of the

500kHz oscillator clock signal sets the RS Flip-Flop. Its

output turns on HS MOSFET. The current sense amplifier

is reset for every switching cycle.

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.

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 EA

increases when feedback voltage (VFB) is lower than the

reference voltage of 0.807V. This also increases the

inductor current as it is proportional to the EA voltage.

If in one cycle the current in the power MOSFET does not

reach the COMP set current value, the power MOSFET

will be forced to turn off. When the HS MOSFET turns off,

the synchronous LS MOSFET turns on until the next clock

cycle begins. There is a “dead time” between the HS turn

off and LS turn on that prevents the switches from

“shooting through” from the input supply to ground.

The voltage loop is internally compensated with the 50pF

and 200k

Ω RC network. The maximum EA voltage output

is precisely clamped at 2.1V.

Internal Regulator

Most of the internal circuitries including the bottom driver

are powered from the 5V internal regulator. This regulator

uses the Vin input to regulate at 5V. When Vin is less than

5V, this internal regulator cannot maintain the 5V

regulation and hence the output voltage would also drop

from regulation.

Enable

The enable (EN) input allows the user to control turning

on or off the regulator. To enable the regulator EN must

be pulled above the ‘EN Rising Threshold’ and to disable

the regulator EN must be pulled below ‘EN falling

Threshold’

(EN

rising

threshold

–

En

threshold

Hysteresis).

Few conditions on EN function:

1) EN must be pulled low for at least 5us to disable the

regulator.

2) The voltage on EN can not exceed 5V.

3) AP6507 can be enabled by Vin through a voltage

divider as shown in the figure 3 below.

Figure 1. EN Divider Network

1M

Ω

||

BOT

R

1M

Ω

||

BOT

R

TOP

(R

RISE

EN

V

RISE

IN

V

+

−

=

−

Where

RISE

EN

= 1.3V(TYP)

1M

Ω

||

BOT

R

1M

Ω

||

BOT

R

TOP

(R

FALL

EN

V

FALL

IN

V

+

−

=

−

Where

FALL

EN

= 0.9V(TYP)

Internal Soft Start

Soft start is traditionally implemented to prevent an

excess inrush current.

This in turn prevents the

converter output voltage from overshooting when it

reaches regulation. The AP6507 has an internal current

source with a soft start capacitor to ramp the reference

voltage from 0V to 0.807V. The soft start time is internally

fixed at 2ms (TYP). The soft start sequence is reset when

there is a Thermal Shutdown, Under Voltage Lockout

(UVLO) or when the part is disabled using the EN pin.