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NJU8753

Analog Signal Input Class D Amplifier with DC-DC Converter for Piezo Speaker

Japan Radio Corporation  

NJU8753

FUNCTIONAL DESCRIPTION

(1) Signal Output

The OUTP and OUTN generate PWM output signals, which will be converted to analog signal via external

2nd-order or higher LC filter. A switching regulator with a high response against a voltage fluctuation is the best

selection for the VDDO, which are the power supply for output drivers. To obtain better THD performance, the

stabilization of the power is required.

(2) EN1

By setting the EN1 pin to “L”, the standby mode is enabled. In the standby mode, the entire functions of the

NJU8753 enter a low-power state, and the output pins (OUTP and OUTN) are high impedance.

(3) Low Voltage Detector

When the power supply voltage drops down to below VDD(MIN), the internal oscillation is halted for prevention

to generate unwanted frequency, and the output pins (OUTP, OUTN) become in high impedance.

(4) Step-up switching regulator

The switching regulator is used as power supply(VDDO) for power amplifier of class-D. The PFM controlled

switching regulator works with external components, which are coil, capacitor, Schottky-diode and resistor for

Step-up voltage.

By setting the EN2 pin to “H”, the step-up operation is enabled, and in case of “L”, standby mode is enabled.

Step-up voltage is set by internal reference voltage(VREG / 2) and external resistors.

The step-up voltage can be calculated by the following methods:

<Calculation of the step-up voltage>

The step-up voltage is determined by internal reference voltage(VREF), R1 and R2. (See Figure.2)

ex. VDD=2.85V, Internal reference voltage(VREF) = VREG / 2=1.425[V], R1=2MΩ, R2=330kΩ

” Step-up voltage[V] = VREF×((R1+R2)/R2) = 1.425×((2M + 330k)/330k) = 10.06[V] ”

Note 1) Apply VREG first, next VBAT. Otherwise, the voltage stress may cause a permanent damage to the IC.

(5) Short Circuit Protection

The short protector, which protects the NJU8753 against high short-circuit current, turns off the output driver.

After about 5 seconds from the protection, the NJU8753 returns to normal operation. The short protector

functions at the following accidents.

-Short between OUTP and OUTN

-Short between OUTP and VSS

-Short between OUTN and VSS

Note 1) The detectable current and the period for the protection depend on the power supply voltage

and ambient temperature.

Note 2) The short protector is not effective for a long term short-circuit current but for an instantaneous accident.

Continuous high-current may cause permanent damage to the NJU8753.

-4-

Ver.2003-09-26

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