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AP3039

BOOST CONTROLLER

BCD Semiconductor 

Data Sheet

BOOST CONTROLLER

AP3039

Application Information

Operation

AP3039 is a boost DC-DC controller with adjustable

operation frequency. Current mode control scheme

provides excellent line and load regulation. Operation

can be best understood by referring to Figure 3.

At the start of each oscillator cycle, the SR latch is set

and external power switch Q1 (see Figure 20) turns on

and the switch current will increase linearly. The

voltage on external sense resistor RCS (see Figure 20),

connected from CS pin to GND, is proportional to the

switch current. This voltage is added to a stabilizing

ramp and the result is fed into the non-inversion input

of the PWM comparator. When this non-inversion

input voltage exceeds inversion input voltage of PWM

comparator which is the output voltage of the error

amplifier EA, the SR latch is reset and the external

power switch turns off. The voltage level at inversion

input of PWM comparator sets the peak current level to

keep the output voltage in regulation. This voltage

level is the amplified signal of the voltage difference

between feedback voltage and reference voltage of

0.5V. So, a constant output current can be provided by

this operation mode.

Input Under-Voltage Detector

AP3039 contains an Under Voltage Lock Out (UVLO)

circuit. Two resistors R1 and R2 are connected from

UVLO pin to ground and VIN pin respectively (see

Figure 20). The resistor divider must be designed such

that the voltage on the UVLO pin is higher than 1.25V

when VIN is in the desired operating range. If the

voltage on the pin is below under voltage threshold, all

functions of AP3039 are disabled, but the system will

remain in a low power standby state. UVLO hysteresis

is accomplished through an internal 22µA current

source which switched on or off 22µA current into the

impedance of the set-point divider. When the UVLO

threshold is exceeded, the current source is activated to

instantly raise the voltage on the UVLO pin. When the

UVLO pin voltage falls below the threshold the current

source is turned off, causing the voltage on the UVLO

pin to fall. The formula for UVLO can be expresses as

blow:

For Input Threshold Voltage

VIN_THRESHOLD=1.25V*(R1+R2)/R2

For Input Hysteresis Voltage

VIN-HYSTERESIS=22µA*R1

Over Voltage Protection

AP3039 has an over voltage protection (OVP) circuit.

The OV Pin is connected to the center tap of R3 and R4

resistor voltage-divider from the high voltage output to

GND (see Figure 20). When the loop is open or the

output voltage becomes excessive in any case, result

the voltage on OV pin exceeds 1.25V, all functions of

AP3039 will be disabled, and the output voltage will

fall. OVP hysteresis is accomplished with an internal

22µA current source and the operation mode is the

same as UVLO. The formula for OVP can be expresses

as blow:

For OVP Voltage

VOVP=1.25V*(R3+R4)/R4

For OVP Hysteresis Voltage

VOVP-HYSTERESIS=22µA*R3

Frequency Selection

An external resistor RT, connected from RT pin to

GND, is used to set the operating frequency (see Figure

20). Operation frequency range is from 200kHz to

1MHz (see Table 1). High frequency operation

optimizes the regulator for the smallest component

size, while low frequency operation can reduce the

switch losses.

Table 1. Frequency Selection

Resistance of RT (kΩ)

390

147

95

68

51

Operating Frequency

(kHz)

200

400

600

800

1000

Dec. 2009 Rev. 1. 4

BCD Semiconductor Manufacturing Limited

13

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