AP6508

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

AP6508

Document number: DS33437 Rev. 4 - 2

10 of 14

www.diodes.com

July 2011

© Diodes Incorporated

External Soft Start

Soft start is traditionally implemented to prevent the

excess inrush current. This in turn prevents the converter

output

voltage

from

overshooting

when

it

reaches

regulation. The AP6508 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 time can be extended > 2ms by

adding a soft start capacitor externally. 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.

External Soft Start can be calculated from the formula

below:

DT

DV

*

C

SS

I

=

Where;

Iss = Soft Start Current

C = External Capacitor

DV=change in output voltage from 0V to maximum voltage

DT = Soft Start Time

Current Limit Protection

The

AP6508

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 AP6508 has a

UVLO comparator that monitors the inout voltage and

internal bandgap reference. If the input voltage falls below

3.8V, the AP6508 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 AP6508 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 is 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%)

10

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

L

IN

OUT

IN

OUT

f

ΔI

V

)

V

(V

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

ΔI

I

I

L

LOAD

L(MAX)

+

=