AP6502

340kHz 23V 2A SYNCHRONOUS DC/DC BUCK CONVERTER

AP6502

Document Number: DS35423 Rev. 2 - 2

10 of 12

www.diodes.com

September 2011

© Diodes Incorporated

Thermal Shutdown

The AP6502 has on-chip thermal protection that prevents

damage to the IC when the die temperature exceeds safe

margins. It implements a thermal sensing to monitor the

operating junction temperature of the IC. Once the die

temperature rises to approximately 150°C, the thermal

protection feature gets activated .The internal thermal

sense circuitry turns the IC off thus preventing the power

switch from damage.

A hysteresis in the thermal sense circuit allows the device

to cool down to approximately 120°C before the IC is

enabled again through soft start. This thermal hysteresis

feature prevents undesirable oscillations of the thermal

protection circuit.

Setting the Output Voltage

The output voltage can be adjusted from 0.925V to 18V

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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

=

1

0.925

OUT

V

2

R

1

R

Figure 4. Feedback Divider Network

When output voltage is low, network as shown in Figure 4

is recommended.

Vout(V)

R1(K

Ω)

R2(K

Ω)

5

45.3

10

3.3

26.1

10

2.5

16.9

10

1.8

9.53

10

1.2

3

10

Table 1—Resistor Selection for Common Output

Voltages

Compensation Components

The AP6502 has an external COMP pin through which

system stability and transient response can be controlled.

COMP pin is the output of the internal trans-conductance

error amplifier. A series capacitor-resistor combination

sets

a

pole-zero

combination

to

control

the

characteristics of the control system. The DC gain of the

voltage feedback loop is given by:

OUT

V

FB

V

VEA

A

CS

G

LOAD

R

VDC

A

×

×

×

=

The control loop transfer function incorporates two poles

one is due to the compensation capacitor (C3) and the

output resistor of error amplifier, and the other is due to

the output capacitor and the load resistor. These poles

are located at:

VEA

A

3

C

2

EA

G

P1

f

×

×

π

=

LOAD

R

2

C

2

1

P2

f

×

×

π

=

One zero is present due to the compensation capacitor

(C3) and the compensation resistor (R3). This zero is

located at:

3

R

3

C

2

1

Z1

f

×

×

π

=

The goal of compensation design is to shape the

converter transfer function to get a desired loop gain. The

system crossover frequency where the feedback loop

has the unity gain is crucial.

A rule of thumb is to set the crossover frequency to below

one-tenth of the switching frequency. Use the following

procedure to optimize the compensation components:

1. Choose the compensation resistor (R3) to set the

desired crossover frequency. Determine the R3 value by

the following equation:

FB

V

OUT

V

EA

G

fs

1

.

0

2

C

2

FB

V

OUT

V

CS

G

EA

G

fc

2

C

2

3

R

CS

G

×

×

×

×

π

<

×

×

×

×

π

=

×

less than one tenth of the switching frequency.