Cont

LTC1436-PLL PIN 1 (MOD)

Figure 152. Switch-Frequency Modulator

LTC1436-PLL PIN 1 (MOD)

Figure 152. Switch-Frequency Modulator

TECHNOLOGY AN84-83

-20dBm

-40dBm

-60dBm

-80dBm

-120dBm

-40dBm

-60dBm

-80dBm

10kHz

Figure 153. Audio Frequencies in Output Noise during Cycle-Skipping Operation

10Hz

10kHz

20kHz

Figure 153. Audio Frequencies in Output Noise during Cycle-Skipping Operation

-20dBm

-40dBm

-60dBm

-80dBm

-100dBm

-120dBm

-40dBm

-60dBm

-80dBm

-100dBm

10kHz

Figure 154. Output Noise with Adaptive Power Operation

10Hz

10kHz

20kHz

Figure 154. Output Noise with Adaptive Power Operation

Audio Frequency Suppression

The Adaptive Power feature of the LTC1436-PLL significantly reduces audio frequency generation, while maintaining good efficiency under very light load conditions. Figure 153 shows the audio frequencies generated by the highly efficient cycle skipping mode of the LTC1436-PLL. Figure 154 shows the decrease in audio frequencies resulting from Adaptive Power operation. Figure 155 shows efficiency curves of both the cycle skipping and Adaptive Power modes along with the traditional, forced continuous mode of operation.

the peak-to-peak inductor current to flow, even under no load conditions. The synchronous buck topology allows the top switch, Q1, to put current into the output capacitor, followed by the bottom switch, Q2, taking current out of the output capacitor while regulating the output voltage under no-load conditions. Although constant frequency is maintained, high current I2R losses and high gate charge losses continue under light load conditions. Forced-current operation is useful for fast transient response required for high di/dt loads like the Intel PentiumĀ® processor.

Cycle skipping is the most efficient mode during light-load operation, where the output capacitor supplies load current most of the time and is replenished by bursts of energy at a rate determined by the load. When load current is low enough, the burst rate falls into the audio-frequency range, which can cause problems. With the addition of Q3, an inexpensive SOT-23 size MOSFET, the Adaptive Power circuitry inside the LTC1436-PLL takes control during light load conditions, turning off high current MOSFETs Q1 and Q2. Q3 and D2 are then used in a conventional constant frequency buck mode, eliminating the power loss caused by charging and discharging the large input capacitance of both power MOSFETs.

The conventional way of avoiding audio-frequency interference is the forced current mode, where both high current MOSFETs continue to operate at full frequency and normal duty cycle under all load conditions. This causes

Cycle skipping, Adaptive Power and forced current operation are all available on the LTC1436-PLL, so that the best operating mode can be selected for each application.

1. CYCLE SKIPPING OPERATION: VARIABLE FREQUENCY COMPONENTS AT LOWER OUTPUT CURRENTS

2. Adaptive Power OPERATION: CONSTANT FREQUENCY WITH AUTOMATIC SWITCHOVER TO SMALL MOSFET Q3

3. FORCED CONTINUOUS OPERATION: CONSTANT FREQUENCY USING LARGE MOSFETS Q1 AND Q2

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