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NCP4894MNR2G Datasheet(PDF) 11 Page - ON Semiconductor |
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NCP4894MNR2G Datasheet(HTML) 11 Page - ON Semiconductor |
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11 / 18 page  NCP4894 http://onsemi.com 11 APPLICATION INFORMATION Detailed Description The NCP4894 audio amplifier can operate under 2.6 V until 5.5 V power supply. It delivers 320 mW rms output power to 4.0 W load (VP = 2.6 V) and 1.0 W rms output power to 8.0 W load (VP = 5.0 V). The structure of the NCP4894 is basically composed of two identical internal power amplifiers. Both are externally configurable with gain−setting resistors Rin and Rf (the closed−loop gain is fixed by the ratios of these resistors). The load is driven differentially through OUTA and OUTB outputs. This configuration eliminates the need for an output coupling capacitor. Internal Power Amplifier The output PMOS and NMOS transistors of the amplifier were designed to deliver the output power of the specifications without clipping. The channel resistance (Ron) of the NMOS and PMOS transistors does not exceed 0.6 W when they drive current. The structure of the internal power amplifier is composed of three symmetrical gain stages, first and medium gain stages are transconductance gain stages to obtain maximum bandwidth and DC gain. Turn−On and Turn−Off Transitions A cycle with a turn−on and turn−off transition is illustrated with plots that show both single ended signals on the previous page. In order to eliminate “pop and click” noises during transitions, output power in the load must be slowly established or cut. When logic high is applied to the shutdown pin, the bypass voltage begins to rise exponentially and once the output DC level is around the common mode voltage, the gain is established slowly (20 ms). Using this turn−on mode, the device is optimized in terms of rejection of “pop and click” noises. A theoretical value of turn−on time at 25 °C is given by the following formula. Cby: bypass capacitor R: internal 150 k resistor with a 25% accuracy Ton = 0.95 * R * Cby The device has the same behavior when it is turned−off by a logic low on the shutdown pin. During the shutdown mode, amplifier outputs are connected to the ground. However, to totally cut the output audio signal, you only need to wait for 20 ms. Shutdown Function The device enters shutdown mode once the SD SELECT and SD MODE pins are in the same logic state. This brings flexibility to the design, as the SD MODE pin must be permanently connected to VP or GND on the PCB. If the SD SELECT pin is not connected to the output of a microcontroller or microprocessor, it’s not advisable to let it float. A pulldown or pullup resistor is then suitable. During the shutdown state, the DC quiescent current has a typical value of 10 nA. Current Limit Circuit The maximum output power of the circuit (Porms = 1.0 W, VP = 5.0 V, RL = 8.0 W) requires a peak current in the load of 500 mA. In order to limit the excessive power dissipation in the load when a short−circuit occurs between both outputs, the current limit in the load is fixed to 800 mA. Thermal Overload Protection Internal amplifiers are switched off when the temperature exceeds 160 °C, and will be switched on again only when the temperature decreases below 140 °C. The NCP4894 is unity−gain stable and requires no external components besides gain−setting resistors, an input coupling capacitor and a proper bypassing capacitor in the typical application. Both internal amplifiers are externally configurable (Rf and Rin) with gain configuration. The differential−ended amplifier presents two major advantages: − The possible output power is four times larger (the output swing is doubled) as compared to a single−ended amplifier under the same conditions. − Output pins (OUTA and OUTB) are biased at the same potential VP/2, this eliminates the need for an output coupling capacitor required with a single−ended amplifier configuration. The differential closed loop−gain of the amplifier is given by Avd + * Rf Rin + Vorms Vinrms . Vorms is the rms value of the voltage seen by the load and Vinrms is the rms value of the input differential signal. Output power delivered to the load is given by Porms + (Vopeak)2 2* RL (Vopeak is the peak differential output voltage). When choosing gain configuration to obtain the desired output power, check that the amplifier is not current limited or clipped. The maximum current which can be delivered to the load is 500 mA Iopeak + Vopeak RL . |
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