LTC3876_15 Datasheet, PDF (29/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
So which mode should be programmed? While either
mode satisfies most practical applications, some trade-
offs exist. The ratiometric mode saves a pair of resistors,
but the coincident mode offers better output regulation.
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
Phase and Frequency Synchronization
For applications that require better control of EMI and
switching noise or have special synchronization needs, the
LTC3876 can synchronize the turn-on of the top MOSFET
to an external clock signal applied to the MODE/PLLIN pin.
The applied clock signal needs to be within ±30% of the
RT programmed frequency to ensure proper frequency
and phase lock. The clock signal levels should generally
comply to V_PLLIN(H) > 2V and V_PLLIN(L) < 0.5V. The MODE/
PLLIN pin has an internal 600k pull-down resistor to ensure
discontinuous current mode operation if the pin is left open.
The LTC3876 uses the voltages on V_IN and V_OUT pins as well
as R_T to adjust the top gate on-time in order to maintain
phase and frequency lock for wide ranges of V_IN, V_OUT
and R_T-programmed switching frequency f:
t_ON
≈
V_OUT
V_IN • f
EXTERNAL
POWER
SUPPLY
EXTERNAL
POWER
SUPPLY
VDDQ
VDDQ
TIME
Coincident Tracking
TIME
3876 F08a
Ratiometric Tracking
Figure 8a. Two Different Modes of Output Tracking
EXTERNAL
POWER
SUPPLY
TO
TRACK/SS1
PIN
VDDQ
R_FB2(1)
R_FB1(1)
R_FB2(1)
TO
V_OUTSENSE1+
R_FB1(1)
PIN
TO
V_{OUTSENSE1–}
PIN
Coincident Tracking Setup
EXTERNAL
POWER
SUPPLY
TO
TRACK/SS1
PIN
R2
TO
V_OUTSENSE1+
PIN
R1
TO
V_{OUTSENSE1–}
PIN
VDDQ
R_FB2(2)
R_FB1(2)
3876 F08b
Ratiometric Tracking Setup
Figure 8b. Setup for Coincident and Ratiometric Tracking
3876f
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