BD8151EFV, BD8157EFV

Technical Note

7/17

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2009.07 - Rev.B

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Description of Operation of Each Block

 Error amp (ERR)

This is the circuit to compare the reference voltage 1.245 V (Typ.) and the feedback voltage of output voltage. The COM

pin voltage resulting from this comparison determines the switching duty. At the time of start, since the soft start is

operated by the SS pin voltage, the COMP pin voltage is limited to the SS pin voltage.

 Oscillator (OSC)

This block generates the oscillating frequency. Either a 600 kHz or 1.2 MHz (Typ.) frequency can be selected with the

FCLK pin.

 SLOPE

This block generates the triangular waveform from the clock generated by OSC. Generated triangular waveform is sent to

the PWM comparator.

 PWM

The COMP pin voltage output by the error amp is compared to the SLOPE block's triangular waveform to determine the

switching duty. Since the switching duty is limited by the maximum duty ratio which is decided internally, it does not

become 100%.

 Reference voltage (VREF)

This block generates the internal reference voltage of 1.245 V (Typ.).

 Protection circuit (UVLO/TSD)

UVLO (under-voltage lockout protection circuit) shuts down the circuits when the voltages are 2.2 V (Typ.BD8151EFV)

1.8 V (Typ.ND8157EFV) or lower. Thermal shutdown circuit shuts down IC at 175°C (Typ.) and recovers at 160°C (Typ.).

 Overcurrent protection circuit (OCP)

Current flowing to the power FET is detected by voltage at the CURRENT SENSE and the overcurrent protection operates

at 3 A (Typ.). When the overcurrent protection operates, switching is turned off and the SS pin capacity is discharged.

 Soft start circuit

Since the output voltage rises gradually while restricting the current at the time of startup, it is possible to prevent the

output voltage overshoot or the inrush current.

 Buffer amp and VCOM

This buffer amp is used to set the gamma correction voltage, which can be set in from 0.2 V to (VOUT - 0.2 V). Use the

VOUT resistance division to set the gamma correction voltage. The VCOM voltage is set similarly.

Fig. 26 Application Circuit Diagram

10uF

Vo

RB161M-20

VCOM

V1

V2

V3

V4

10uH

10uF

VCC

2.5V

COMIN

13

12

11

6

7

8

9

10

2

3

4

5

1

SLOPE

OSC

UVLO

TSD

+

SET

RESET SDWN

LOGIC

CURRENT

SENSE

+

-

ERR

20

19

18

16

15

14

17

BUFFER SUPPLY

1.245V

SW

VCC

ENB

FCLK

VS

IN1

IN2

IN3

IN4

PGND

GND

FB

COMP

SS

VCOM

OUT1

OUT2

OUT3

OUT4

PWM

+ -

OCP

DRV

START

SOFT

TOP VIEW

20kΩ

20kΩ

20kΩ

20kΩ

20kΩ

20kΩ

20kΩ

100kΩ

15kΩ

5.1kΩ 3300pF

0.01uF