LTC1436A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1436A' PDF : 28 Pages View PDF
LTC1436A
LTC1436-PLL-A/LTC1437A
APPLICATIONS INFORMATION
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
1.3fO
fO
0.7fO
0
0.5
1.0
1.5 2.0
2.5
VPLLLPF (V)
1436 F07
Figure 7. Operating Frequency vs VPLLLPF
EXTERNAL
FREQUENCY
2.4V
PHASE
DETECTOR
PLLIN
DIGITAL
PHASE/
FREQUENCY
50k
DETECTOR
RLP
CLP
COSC
PLL LPF
COSC
OSC
1436 F08
Figure 8. Phase-Locked Loop Block Diagram
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
0.1µF. Be sure to connect the low side of the filter to SGND.
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.
PLL LPF
2.4V MAX
3.3V OR 5V
18k
1436 F09
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:
VLBTRIP
=
1.19V
1+
R4
R3
VIN
SGND
R4
LBI –
LTC1436A
LTC1437A
LBO
R3
+
1.19V REFERENCE
1436 F10
Figure 10. Low Battery Comparator
18
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