AP6508

500kHz 21V 3A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6508

Document number: DS33437 Rev. 4 - 2

9 of 14

www.diodes.com

July 2011

© Diodes Incorporated

Application Information

Theory of Operation

The AP6508 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. Figure. 2 depicts the functional block diagram

of AP6508.

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 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 Error

Amplifier 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 520ns “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 EAMP 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 atleast 5us to disable the

regulator.

2) The voltage on EN cannot exceed 5V.

3) AP6508 can be enabled by Vin through a voltage

divider as shown in the figure 1 below.

Figure 1. EN Divider Network

1M

Ω

||

R

1M

Ω

||

R

(R

V

V

BOT

BOT

TOP

RISE

EN

RISE

IN

+

=

−

−

Where

RISE

EN

V

−

= 1.3V(TYP)

1M

Ω

||

R

1M

Ω

||

R

(R

V

V

BOT

BOT

TOP

FALL

EN

FALL

IN

+

=

−

−

Where

FALL

EN

V

−

= 0.9V(TYP)

Power Good

Power Good is an open drain and active high output. This

output can be pulled up high to the appropriate level with

an external resistor. The PGOOD is flagged low when

Vfb=0.7V and is an open drain output when Vfb=0.9V.

The PGOOD output can deliver a max of 4 mA sink

current at 0.4 V when de- asserted. The PGOOD pin is

held low during soft-start. Once output voltage reaches

90% of its final value, PGOOD goes high if there are no

faults.