ltc3862gn-1 Linear Technology Corporation, ltc3862gn-1 Datasheet - Page 17

• Manufacturer Part Number: ltc3862gn-1
• Description: Multi-phase Current Mode Step-up Dc/dc Controller
• Manufacturer: Linear Technology Corporation

The LTC3862-1 uses a constant frequency architecture that
can be programmed over a 75kHz to 500kHz range using
a single resistor from the FREQ pin to ground. Figure 6
illustrates the relationship between the FREQ pin resistance
and the operating frequency.
The operating frequency of the LTC3862-1 can be ap-
proximated using the following formula:
A phase-lock loop is available on the LTC3862-1 to syn-
chronize the internal oscillator to an external clock source
connected to the SYNC pin. Connect a series RC network
from the PLLFLTR pin to SGND to compensate PLL’s
feedback loop. Typical compensation components are a
0.01μF capacitor in series with a 10k resistor. The PLLFLTR
pin is both the output of the phase detector and the input
to the voltage controlled oscillator (VCO). The LTC3862-1
phase detector adjusts the voltage on the PLLFLTR pin
to align the rising edge of GATE1 to the leading edge of
the external clock signal, as shown in Figure 7. The ris-
ing edge of GATE2 will depend upon the voltage on the
PHASEMODE pin. The capture range of the LTC3862-1’s
PLL is 50kHz to 650kHz.
Because the operating frequency of the LTC3862-1 can be
programmed using an external resistor, in synchronized
applications, it is recommended that the free-running fre-
quency (as defi ned by the external resistor) be set to the
same value as the synchronized frequency. This results in
a start-up of the IC at approximately the same frequency
as the external clock, so that when the sync signal comes
alive, no discontinuity at the output will be observed. It also
ensures that the operating frequency remains essentially
constant in the event the sync signal is lost. The SYNC
pin has an internal 50k resistor to ground.
Using the CLKOUT and PHASEMODE Pins
in Multi-Phase Applications
The LTC3862-1 features two pins (CLKOUT and PHASE-
MODE) that allow multiple ICs to be daisy-chained together
for higher current multi-phase applications. For a 3- or
4-phase design, the CLKOUT signal of the master controller
OPERATION
R
FREQ
= 5.5096E9(f
OSC
)
–0.9255
is connected to the SYNC input of the slave controller in
order to synchronize additional power stages for a single
high current output. The PHASEMODE pin is used to adjust
the phase relationship between channel 1 and channel
2, as well as the phase relationship between channel 1
and CLKOUT, as summarized in Table 1. The phases are
calculated relative to the zero degrees, defi ned as the ris-
ing edge of the GATE1 output. In a 6-phase application
the CLKOUT pin of the master controller connects to the
SYNC input of the 2nd controller and the CLKOUT pin of
the 2nd controller connects to the SYNC pin of the 3rd
controller.
CLKOUT
10V/DIV
20V/DIV
20V/DIV
10V/DIV
GATE1
GATE2
SYNC
Figure 6. FREQ Pin Resistor Value vs Frequency
Figure 7: Synchronization of the LTC3862-1
to an External Clock Using the PLL
1000
100
V
V
I
PHASEMODE = SGND
10
OUT
IN
OUT
0
= 24V
= 0.5A
= 72V
100
200
300
FREQUENCY (kHz)
400
2μs/DIV
500
600
700
LTC3862-1
800
900
38621 F06
1000
38621 F07
17
38621f