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LTC1436A

LTC1436-PLL-A/LTC1437A

APPLICATIONS INFORMATION

difference. Thus the voltage on the PLL LPF pin is adjusted

until the phase and frequency of the external and internal

oscillators are identical. At this stable operating point the

phase comparator output is open and the filter capacitor

CLP holds the voltage.

The loop filter components CLP and RLP smooth out the

current pulses from the phase detector and provide a

stable input to the voltage-controlled oscillator. The filter

components CLP and RLP determine how fast the loop

acquires lock. Typically, RLP = 10k and CLP is 0.01µF to

0.1

µF.BesuretoconnectthelowsideofthefiltertoSGND.

The PLL LPF pin can be driven with external logic to obtain

a 1:1.9 frequency shift. The circuit shown in Figure 9 will

provide a frequency shift from fO to 1.9fO as the voltage

and VPLLLPF increases from 0V to 2.4V. Do not exceed 2.4V

on VPLLLPF.

The output of the phase detector is a complementary pair

of current sources charging or discharging the external

filter network on the PLL LPF pin. The relationship

between the PLL LPF pin and operating frequency is

shown in Figure 7. A simplified block diagram is shown in

Figure 8.

If the external frequency (fPLLIN) is greater than the oscil-

lator frequency (f), current is sourced continuously, pull-

ing up the PLL LPF pin. When the external frequency is less

than fOSC, current is sunk continuously, pulling down the

PLL LPF pin. If the external and internal frequencies are the

same but exhibit a phase difference, the current sources

turn on for an amount of time corresponding to the phase

Figure 7. Operating Frequency vs VPLLLPF

PLL LPF

2.4V MAX

3.3V OR 5V

1436 F09

18k

Figure 9. Directly Driving PLL LPF Pin

Low-Battery Comparator

The LTC1436A/LTC1437A have an on-chip low-battery

comparator which can be used to sense a low-battery

condition when implemented as shown in Figure 10. The

resistive divider R3, R4 sets the comparator trip point as

follows:

LBTRIP =+









119

PLLIN

50k

1436 F08

PLL LPF

COSC

PHASE

DETECTOR

OSC

RLP

CLP

COSC

EXTERNAL

FREQUENCY

2.4V

DIGITAL

PHASE/

FREQUENCY

DETECTOR

VPLLLPF (V)

1.3fO

0.7fO

1436 F07

1.5

2.0

1.0

0.5

2.5

Figure 8. Phase-Locked Loop Block Diagram

Figure 10. Low Battery Comparator

–

LBI

VIN

SGND

LBO

1436 F10

1.19V REFERENCE

LTC1436A

LTC1437A

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