LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 11

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

OPERATION
DRV
Power for the top and bottom MOSFET drivers is derived
from the DRV
from a fl oating bootstrap capacitor, C
is normally recharged from DRV
Schottky diode, D
2-Phase Operation
For the LTC3708 to operate optimally as a 2-phase controller,
the resistors connected to the I
such that the free-running frequency of each channel is
close to that of the other. An internal phase-locked loop
(PLL) will then ensure that channel 2 operates at the same
frequency as channel 1, but phase shifted by 180°. The
loop fi lter connected to the INTLPF pin provides stability to
the PLL. For external clock synchronization, a second PLL
is incorporated to adjust the on-time of channel 1 until its
frequency is the same as the external clock. Compensation
for the external PLL is through the EXTLPF pin.
The loop fi lter components tied to the INTLPF and EXTLPF
pins are used to compensate the internal PPL and external
PLL respectively. The typical value ranges are:
For noise suppression, a capacitor with a value of 1nF or
less should be placed from INTLPF to ground and EXTLPF
to ground.
INTLPF: R
EXTLPF: R
CC
IPLL
EPLL
CC
pin. The top MOSFET driver is powered
= 2kΩ to 10kΩ, C
B
≤ 1kΩ, C
, when the top MOSFET is turned off.
Figure 1. Input Waveforms Comparing Single Phase (1a) and 2-Phase (1b) Operation
for Dual Switching Regulators Converting 12V to 5V and 3.3V at 3A Each
(Refer to Functional Diagram)
I
EPLL
IN(MEAS)
ON
(1a)
= 10nF to 100nF
= 2.53A
CC
pins must be selected
IPLL
through an external
RMS
B
= 10nF to 100nF
. This capacitor
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
The LTC3708’s 2-phase operation brings considerable
benefi ts to portable applications and automatic electron-
ics. It lowers the input fi ltering requirement, reduces
electromagnetic interference (EMI) and increases the
power conversion effi ciency. Until the introduction of the
2-phase operation, dual switching regulators operated
both channels in phase (i.e., single phase operation).
This means that both controlling switches turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor or battery. Such operation results in higher
input RMS current, larger and/or more expensive input
capacitors, more power loss and worse EMI in the input
source (whether a wall adapter or a battery).
In contrast to single phase operation, the two channels of
a 2-phase switching regulator are operated 180 degrees
out of phase. This effectively interleaves the current pulses
drawn by the switches, greatly reducing the overlap time
where they add together. The result is a signifi cant reduc-
tion in total RMS input current, which in turn allows less
expensive input capacitors to be used, reduces shielding
requirements for EMI and improves real world operating
effi ciency.
Figure 1 compares the input waveforms for a representative
single phase dual switching regulator to the 2-phase dual
switching regulator. An actual measurement of the RMS
input current under these conditions shows that 2-phase
dropped the input current from 2.53A
I
IN(MEAS)
= 1.55A
(1b)
RMS
3708 F01
RMS
LTC3708
to 1.55A
11
RMS
3708fb
.