Datasheet

LTC3822-1
15
38221f
over temperature and process variations, to be between
250kHz and 750kHz. A simplifi ed block diagram is shown in
Figure 6.
If the external clock frequency is greater than the internal
oscillator’s frequency, f
OSC
, then current is sourced con-
tinuously from the phase detector output, pulling up the
PLLLPF pin. When the external clock frequency is less
than f
OSC
, current is sunk continuously, pulling down
the PLLLPF 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 difference. The voltage on the PLLLPF pin is
adjusted until the phase and frequency of the internal and
external oscillators are identical. At the stable operating
point, the phase detector output is high impedance and
the fi lter capacitor C
LP
holds the voltage.
The loop fi lter components, C
LP
and R
LP
, smooth out
the current pulses from the phase detector and provide a
stable input to the voltage-controlled oscillator. These fi lter
components determine how fast the loop acquires lock.
Typically R
LP
= 10k and C
LP
is 2200pF to 0.01µF.
Typically, the external clock (on SYNC/MODE pin) input
high level is 1.6V, while the input low level is 1.2V.
Table 2 summarizes the different states in which the
PLLLPF pin can be used.
Table 2. The States of the PLLLPF Pin
PLLLPF PIN SYNC/MODE PIN FREQUENCY
0V DC Voltage 300kHz
Floating DC Voltage 550kHz
V
IN
DC Voltage 750kHz
RC Loop Filter Clock Signal Phase-Locked
to External Clock
Using a Sense Resistor (GN Only)
A sense resistor R
SENSE
can be connected between V
IN
and
SENSE
–
to sense the output load current. In this case, the
drain of the topside N-channel MOSFET is connected to
SENSE
–
pin and the source is connected to the SW pin of
the LTC3822-1. Therefore the current comparator moni-
tors the voltage developed across R
SENSE
, not the V
DS
of
the top MOSFET. The output current that the LTC3822-1
can provide in this case is given by:
I
V
R
I
OUT MAX
SENSE MAX
SENSE
RIPPLE
()
()
–=
∆
2
Setting ripple current as 40% of I
OUT(MAX)
and using
Figure 1 to choose SF, the value of R
SENSE
is:
RSF
V
I
SENSE
SENSE MAX
OUT MAX
=
∆
5
6
••
()
()
Variation in the resistance of a sense resistor is much
smaller than the variation in on-resistance of an external
MOSFET. Therefore the load current is well controlled
with a sense resistor. However the sense resistor causes
I
2
R losses in addition to those of the MOSFET. Therefore,
using a sense resistor lowers the effi ciency of LTC3822-1,
especially at high load current.
APPLICATIONS INFORMATION
PLLLPF PIN VOLTAGE (V)
0.2
0
FREQUENCY (kHz)
0.7 1.2 1.7
38221 F05
2.2
200
400
600
800
1000
1200
DIGITAL
PHASE/
FREQUENCY
DETECTOR
OSCILLATOR
2.4V
R
LP
C
LP
38221 F06
PLLLPF
EXTERNAL
OSCILLATOR
SYNC/
MODE
Figure 5. Relationship Between Oscillator Frequency and Voltage
at the PLLLPF Pin When Synchronizing to an External Clock
Figure 6. Phase-Locked Loop Block Diagram