Datasheet

LTC3860
14
3860fc
APPLICATIONS INFORMATION
Setting the Output Voltage
The LTC3860 regulates the FB pins to 0.6V. FB is con-
nected to V
OUT
or V
SNSOUT
(for remote output sensing)
via an external resistive divider as shown in Figure 3. The
divider sets the output voltage according to the following
equation:
V
OUT
= 0.6V • 1+
R
B
R
A
⎛
⎝
⎜
⎞
⎠
⎟
Care should be taken to place the output divider resistors
and the compensation components as close as possible
to the FB pin to minimize switching noise coupling into
the control signal path.
Programming the Operating Frequency
The LTC3860 can be hard wired to one of two fi xed fre-
quencies, linearly programmed to any frequency between
250kHz and 1.25MHz or synchronized to an external
clock.
Table 1 in the Operation section shows how to connect the
CLKIN and FREQ pins to choose the mode of frequency
programming. In linear programming mode the frequency
of operation is given by the following equation:
Frequency (R
FREQ
– 15kΩ) • 20Hz/Ω
^

Figure 4 shows operating frequency vs R
FREQ
.
COMP
LTC3860
FB
R
A
R
B
V
OUT
DIVIDER AND COMPENSATION
COMPONENTS PLACED NEAR
FB, SGND AND COMP PINS
C
OUT
3860 F03
SGND
Figure 3. Output Divider and Compensation
Component Placement
Sensing the Output Voltage with a
Differential Amplifi er
When using the remote sense differential amplifi er, care
should be taken to route the V
SNSP
and V
SNSN
PCB traces
parallel to each other all the way to the terminals of the
output capacitor or remote sensing points on the board.
In addition, avoid routing these sensitive traces near any
high speed switching nodes in the circuit. Ideally, they
should be shielded by a low impedance ground plane to
maintain signal integrity.
When using a single LTC3860 to regulate two output
voltages, the negative terminal of V
OUT2
should be
kelvin-connected to SGND and the differential amplifi er
should be used to remotely sense V
OUT1
. This will maxi-
mize output voltage accuracy for both channels.
Figure 4. Operating Frequency vs R
FREQ
Frequency Synchronization
The LTC3860 incorporates an internal phase-locked loop
(PLL) which enables synchronization of the internal os-
cillator (rising edge of PWM1) to an external clock from
250kHz to 1.25MHz.
Since the entire PLL is internal to the LTC3860, simply
applying a CMOS level clock signal to the CLKIN pin will
enable frequency synchronization. A resistor from FREQ
to GND is still required to set the free running frequency
close to the sync input frequency.
R
FREQ
(kΩ)
0
OSCILLATOR FREQUENCY (MHz)
0.7
0.9
1.1
60
100
3860 F04
0.5
0.3
0.1
20 40 80
1.3
1.5
1.7
120