Datasheet

LTC3876
29
3876f
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
APPLICATIONS INFORMATION
Figure 8a. Two Different Modes of Output Tracking
Figure 8b. Setup for Coincident and Ratiometric Tracking
TIME
Coincident Tracking
EXTERNAL
POWER
SUPPLY
VDDQ
OUTPUT VOLTAGE
EXTERNAL
POWER
SUPPLY
VDDQ
TIME
3876 F08a
Ratiometric Tracking
OUTPUT VOLTAGE
Coincident Tracking Setup
R
FB2(1)
R
FB1(1)
R
FB2(1)
R
FB1(1)
TO
V
OUTSENSE1
+
PIN
TO
V
OUTSENSE1
–
PIN
TO
TRACK/SS1
PIN
EXTERNAL
POWER
SUPPLY
VDDQ
Ratiometric Tracking Setup
R2
R1
TO
TRACK/SS1
PIN
EXTERNAL
POWER
SUPPLY
R
FB2(2)
VDDQ
R
FB1(2)
3876 F08b
TO
V
OUTSENSE1
+
PIN
TO
V
OUTSENSE1
–
PIN