Rainbow-electronics MAX1821 Manual do Utilizador descarregar pdf (Página 10)

MAX1820/MAX1821

WCDMA Cellular Phone 600mA

Buck Regulators

10 ______________________________________________________________________________________

ing each cycle, using the PWM comparator. The power

transferred to the load is adjusted by changes in the

inductor peak current limit during the first half of each

cycle, based on the output error voltage.

A new cycle begins at each falling edge of the internal

oscillator. The controller turns on the P-channel MOS-

FET to increase the inductor current, and the slope

compensation block initiates a new reference current

ramp that is summed with the internal P-channel MOS-

FET current (Figures 1 and 2).

The second half of the cycle begins when the reference

ramp is greater than the error voltage. The P-channel

MOSFET is turned off, the synchronous rectifier is

turned on, and inductor current continues to flow to the

output capacitor. The output capacitor stores charge

when the current is high and releases it when the

inductor current is low, smoothing the voltage across

VOLTAGE

REFERENCE

PWM CONTROL

AND

SKIP LOGIC

ERROR SIGNAL

SLOPE COMP

CURRENT SENSE

SKIP THRESHOLD

TRANSIMPEDANCE

ERROR AMP

MAX1820

SKIP

COMPARATOR

PWM

COMPARATOR

PGND

÷13 OR

÷18

1.25V

IC BIAS

0.45V TO 2.15V

1MHz

OSCILLATOR

CLAMP

COMP

SYNC

SKIP

SHDN

GND

BATT

OUT

REF

Figure 1. MAX1820 Simplified Functional Diagram (No SYNC for MAX1820Z)

VOLTAGE

REFERENCE

PWM CONTROL

AND

SKIP LOGIC

ERROR SIGNAL

SLOPE COMP

CURRENT SENSE

SKIP THRESHOLD

TRANSIMPEDANCE

ERROR AMP

MAX1821

SKIP

COMPARATOR

PWM

COMPARATOR

PGND

1.25V

IC BIAS

0.45V TO 2.15V

1MHz

OSCILLATOR

CLAMP

COMP

SYNC

SKIP

SHDN

GND

BATT

OUT

REF

Figure 2. MAX1821 Simplified Functional Diagram (No SYNC for MAX1821)

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 16 17 18

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