12

LTC1433/LTC1434

Figure 10. Phase-Locked Loop Block Diagram

PLLIN

50k

1433/34 F10

PLL LPF

PHASE

DETECTOR

OSC

EXTERNAL

FREQUENCY

2.4V

DIGITAL

PHASE/

FREQUENCY

DETECTOR

0

1433/34 F09

1.5

2.0

1.0

0.5

2.5

Figure 9. Relationship Between Oscillator Frequency

and Voltage at PLL LPF Pin

the PLL LPF pin. When the external frequency is less than

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 differ-

ence. 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

current pulses from the phase detector and provide a

stable input to the voltage controlled oscillator. The filter

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 11 will

Figure 11. Directly Driving PLL LPF Pin

PLL LPF

2.4V MAX

3.3V OR 5V

1433/34 F11

18k

Low-Battery Comparator

The LTC1433/LTC1434 have an on-chip, low-battery com-

parator which can be used to sense a low-battery condi-

tion when implemented as shown in Figure 12. The resis-

tor divider R3/R4 sets the comparator trip point as follows:

V

R

R









119

4

3

1

.

–

+

R4

R3

1433/34 F12

1.19V REFERENCE

LTC1433/LTC1434

Figure 12. Low-Battery Comparator

The divided down voltage at the negative (–) input to the

comparator is compared to an internal 1.19V reference. A

40mV hysteresis is built in to assure rapid switching. The

output is an open-drain MOSFET and requires a pull-up

resistor to operate. This comparator is active in shutdown.

To save more shutdown quiescent current, this compara-

tor can be shut down by taking the LBI pin below 0.74V,