ML2731

7

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

CRYSTAL OSCILLATOR

The crystal oscillator circuit is powered by an on chip

regulator and generates a square wave signal and a sine

wave signal. The sine wave is intended for radio PLL

circuits, and the square wave clock for the digital

circuits.

The frequency range of operation is 10 to 40MHz

(determined by the external crystal). The crystal oscillator

can also be overdriven by connecting an external

oscillator to pin 15 (XIN).

The crystal oscillator square wave and sine wave signal

outputs are enabled in STANDBY, PRETX and TRANSMIT

modes. (See Figures 2 and 3)

The oscillator circuit is enabled by the PDN logic control

pin and turns on and generates stable output signals

within 20mS of being enabled.

Figure 2: Typical 32 MHz Square wave output signal

(1k

W+10pF load, VCC = 3.3V)

Figure 3: Typical 32 MHz sine wave output signal (1k

W+0pF load, and

1k

W+10pF load, VCC = 3.3V)

NEGATIVE VOLTAGE GENERATOR

When driven by a 32 MHz crystal, the charge pump of the

ML2731 is clocked with a 2.66 MHz signal. Because the

clock frequency is so high, the negative voltage generator

requires only small ceramic capacitors on C1 and C2 for

operation. (Figure 4) Typically they have values of 100nF.

The quick response time (Figure 5), due in part to the

small circuit capacitance, allows the charge pump to be

disabled when not needed thus reducing power

consumption.

Figure 5: Typical negative voltage generator on and off timing

(using 100nF capacitors on pins C1 and C2)

FUNCTIONAL DESCRIPTION

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

–0.5

TIME (ns)

010

30

50

40

20

TIME (ns)

0

60

40

80

20

1.9

1.8

1.7

1.6

1.5

1.4

10

70

50

30

Cap:

10pF

none

Figure 4: Typical C1 and C2 Noise Shunt Configuration

NEGEN (pin 7) enables the negative voltage generator

during the PRETX and TRANSMIT modes of operation.

Output from the negative voltage generator is used

internally by the ML2731 to supply the op amp in the PA

current control circuit. The resulting voltage is applied to

VBIAS for controlling an external PA. The negative voltage

generator enables the ML2731 to provide a typical bias

voltage in the range of –2.9 V to 0V.

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

TIME (ms)

00.8

0.6

0.4

0.2

0.1

0.7

0.5

0.3

0.5

0.0

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

–3.5

VNEG

BIAS

VNEG

C1

C2

12

11

10

100nF

100nF

100nF

Figure 6: Negative voltage output ripple (

mV) (using 100nF

bypass and reservoir capacitance)

600

500

400

300

200

100

0

FREQUENCY (MHz)

04

6

10

8

2