Ap3440 – Diodes AP3440 User Manual

Page 8

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AP3440

Document number: DS36691 Rev. 2 - 2

8 of 15

www.diodes.com

March 2014

© Diodes Incorporated

AP3440

A Product Line of

Diodes Incorporated

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Application Note

Theory of Operation

The AP3440 consists of a reference voltage module, slope compensation circuit, error amplifier, PWM comparator, current limit circuit, two N-

channel MOSFETs etc. (Refer to the Functional Block Diagram on page 3 for detailed information)

Soft-start

The AP3440 integrates an internal soft start circuit to minimize inrush currents or provide power supply sequencing during power up. A capacitor

connected between SS pin and ground implements the soft-start time. The AP3440 has an internal pull-up current source of 2

μA, which charges

the external slow start capacitor. Equation 1 calculates the required slow start capacitor, I

SS

is the internal slow start charging current of 2

μA, and

V

REF

is the internal voltage reference of 0.803V.


)

(

)

(

)

(

)

(

V

V

A

I

ms

t

nF

C

REF

SS

SS

SS

m

………………….(1)

During normal operation, if the V

IN

goes below the UVLO, or the EN pin is pulled below 1.2V, or a thermal shutdown occurs, the AP3440 will stop

switching and the SS pin will be discharged to 40mV before reinitiating a powering up sequence.

Enable and Adjusting UVLO

The AP3440 are disabled when the V

IN

falls below 2.6V. If an application requires a higher under-voltage lockout (UVLO), use the EN pin as

shown in Figure 1

to adjust the input voltage UVLO by using two external resistors. The EN pin has an internal pull-up current source that provides

the default condition of the AP3440 operating when the EN pin floats. Once the EN pin voltage exceeds 1.25V, an additional 2.55

μA of hysteresis

is added. When the EN pin is pulled below 1.18V, the 2.55

μA hysteresis is removed. This additional current facilitates input voltage hysteresis.

For AP3440, the divider resistor R1 and R2 on the EN pin can be calculated according to equation 2 and 3.

6

10

59

.

2

944

.

0

1

STOP

START

V

V

R

…………………...(2)


6

10

2

.

3

1

18

.

1

1

18

.

1

2

R

V

R

R

STOP

………………(3)

AP3440

R1

R2

VIN

EN

2.55

μA

0.6

μA

Figure 1. Adjustable Under Voltage Lock Out

Adjusting Output Voltage

The output voltage is set with a resistor divider from the FB pin. It is recommended to use divider resistors with 1% tolerance or better. Start with a

10k

Ω R2 resistor and use the equation 4 to calculate R1. To improve efficiency at very light loads consider using larger value resistors. If the

values are too high, the regulator is more susceptible to noise and voltage errors from the FB input current are noticeable.

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