ML2731CT Micro Linear, ML2731CT Datasheet - Page 7

• Manufacturer Part Number: ML2731CT
• Description: PA Bias Controller
• Manufacturer: Micro Linear

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 3: Typical 32 MHz sine wave output signal (1k +0pF load, and
FUNCTIONAL DESCRIPTION
Figure 2: Typical 32 MHz Square wave output signal
–0.5
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.9
1.8
1.7
1.6
1.5
1.4
0
0
(1k +10pF load, VCC = 3.3V)
10
1k +10pF load, VCC = 3.3V)
10
20
10pF
none
Cap:
20
30
TIME (ns)
TIME (ns)
40
30
50
40
60
January, 2000
PRELIMINARY
70
50
80
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.
PRELIMINARY DATASHEET
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.
Figure 5: Typical negative voltage generator on and off timing
Figure 4: Typical C1 and C2 Noise Shunt Configuration
Figure 6: Negative voltage output ripple ( V) (using 100nF
100nF
(using 100nF capacitors on pins C1 and C2)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
100nF
600
500
400
300
200
100
0
0
0
0
0.1
bypass and reservoir capacitance)
100nF
V NEG
BIAS
0.2
2
0.3
FREQUENCY (MHz)
TIME (ms)
4
0.4
0.5
10
11
12
6
0.6
8
0.7
VNEG
C2
C1
0.8
ML2731
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
10
0.5
0.0
7