ML2731
PRELIMINARY
FUNCTIONAL DESCRIPTION
CRYSTAL OSCILLATOR
NEGATIVEVOLTAGE GENERATOR
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.
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.
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).
VNEG
C2
10
11
12
100nF
The crystal oscillator square wave and sine wave signal
outputs are enabled in STANDBY, PRETX and TRANSMIT
modes. (See Figures 2 and 3)
100nF
C1
100nF
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 4: Typical C1 and C2 Noise Shunt Configuration
NEGEN (pin 7) enables the negative voltage generator
during the PRETX and TRANSMIT modes of operation.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
–0.5
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.
0.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.0
V
NEG
BIAS
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
0
10
20
30
40
50
TIME (ns)
Figure 2: Typical 32 MHz Square wave output signal
(1kW+10pF load, VCC = 3.3V)
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
1.9
TIME (ms)
Cap:
10pF
none
Figure 5: Typical negative voltage generator on and off timing
(using 100nF capacitors on pins C1 and C2)
1.8
600
500
400
300
200
100
0
1.7
1.6
1.5
1.4
0
10
20
30
40
50
60
70
80
0
2
4
6
8
10
TIME (ns)
FREQUENCY (MHz)
Figure 3: Typical 32 MHz sine wave output signal (1kW+0pF load, and
1kW+10pF load, VCC = 3.3V)
Figure 6: Negative voltage output ripple (mV) (using 100nF
bypass and reservoir capacitance)
January, 2000
7
PRELIMINARY DATASHEET
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
