AP6507

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

AP6507

Document number: DS33435 Rev. 2 - 2

10 of 13

www.diodes.com

May 2011

© Diodes Incorporated

Current Limit Protection

In order to reduce the total power dissipation and to

protect the application, AP6507 has cycle-by-cycle current

limiting implementation. The voltage drop across the

internal high-side MOSFET is sensed and compared with

the internally set current limit threshold. This voltage drop

is sensed at about 30ns after the HS turns on. When the

peak inductor current exceeds the set current limit

threshold, current limit protection is activated. During this

time the feedback voltage (VFB) drops down. When the

voltage at the FB pin reaches 0.3V, the internal oscillator

shifts the frequency from the normal operating frequency

of 500kHz to a fold-back frequency of 150kHz. The current

limit is reduced to 70% of nominal current limit when the

part is operating at 150kHz. This low Fold-back frequency

prevents runaway current.

Under Voltage Lockout (UVLO)

Under Voltage Lockout is implemented to prevent the IC

from insufficient input voltages. The AP6507 has a UVLO

comparator that monitors the input voltage and the internal

bandgap reference. If the input voltage falls below 4.0V,

the AP6507 will latch an under voltage fault. In this event

the output will be pulled low and power has to be re-cycled

to reset the UVLO fault.

Thermal Shutdown

The AP6507 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 140°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 0.81V 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:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

=

1

0.81

OUT

V

2

R

1

R

Figure 2. Feedback Divider Network

When output voltage is low, T-type network as shown in

Figure 2 recommended.

1.2

4.99

10.2

24.9

1.8

4.99 (1%)

4.02 (1%)

24.9

2.5

40.2 (1%)

19.1 (1%)

0

3.3

40.2 (1%)

13 (1%)

0

5

40.2 (1%)

7.68 (1%)

35.7

Table 1—Resistor Selection for Common Output

Voltages

Inductor

Calculating the inductor value is a critical factor in

designing a buck converter. For most designs, the

following equation can be used to calculate the inductor

value;

SW

f

L

ΔI

IN

V

)

OUT

V

IN

(V

OUT

V

L

⋅

⋅

−

⋅

=

Where

L

And

SW

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% of the

maximum load current. The maximum inductor peak

current is calculated from:

2

L

ΔI

LOAD

I

L(MAX)

I

+

=