LTC3708_15 Datasheet, PDF (11/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
OPERATION (Refer to Functional Diagram)
DRV_CC
Power for the top and bottom MOSFET drivers is derived
from the DRV_CC pin. The top MOSFET driver is powered
from a floating bootstrap capacitor, C_B. This capacitor
is normally recharged from DRV_CC through an external
Schottky diode, D_B, when the top MOSFET is turned off.
2-Phase Operation
For the LTC3708 to operate optimally as a 2-phase controller,
the resistors connected to the I_ON pins must be selected
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 filter 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 filter components tied to the INTLPF and EXTLPF
pins are used to compensate the internal PPL and external
PLL respectively. The typical value ranges are:
INTLPF: R_IPLL = 2kΩ to 10kΩ, C_IPLL = 10nF to 100nF
EXTLPF: R_EPLL ≤ 1kΩ, C_EPLL = 10nF to 100nF
For noise suppression, a capacitor with a value of 1nF or
less should be placed from INTLPF to ground and EXTLPF
to ground.
The LTC3708’s 2-phase operation brings considerable
benefits to portable applications and automatic electron-
ics. It lowers the input filtering requirement, reduces
electromagnetic interference (EMI) and increases the
power conversion efficiency. 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 significant 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
efficiency.
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_RMS to 1.55A_RMS.
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
I_IN(MEAS) = 2.53A_RMS
(1a)
I_IN(MEAS) = 1.55A_RMS
(1b)
3708 F01
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
3708fb
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