LTC3836_15 Datasheet, PDF (18/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

LTC3836
APPLICATIONS INFORMATION
During soft-start, the start-up of V_OUT1 is controlled by
slowly ramping the positive reference to the error amplifier
from 0V to 0.6V, allowing V_OUT1 to rise smoothly from 0V
to its final value. The default internal soft-start time is 1ms.
This can be increased by placing a capacitor between the
RUN/SS pin and SGND. In this case, the soft-start time
will be approximately:
t_SS1
=
C_SS
•
600mV
0.65μA
Tracking
The start-up of V_OUT2 is controlled by the voltage on the
TRACK/SS2 pin. Normally this pin is used to allow the start-
up of V_OUT2 to track that of V_OUT1 as shown qualitatively
in Figures 7a and 7b. When the voltage on the TRACK/SS2
pin is less than the internal 0.6V reference, the LTC3836
regulates the V_FB2 voltage to the TRACK/SS2 pin voltage
instead of 0.6V. The start-up of V_OUT2 may ratiometrically
track that of V_OUT1, according to a ratio set by a resistor
divider (Figure 7c):
V_{OUT1 =} R2A _• R_TRACKA + R_TRACKB
V_OUT2 R_TRACKA
R2B + R2A
For coincident tracking (V_OUT1 = V_OUT2 during start-up),
R2A = R_TRACKA
R2B = R_TRACKB
The ramp time for V_OUT2 to rise from 0V to its final
value is:
t_SS2
=
t_SS1 •
0.6
V_OUT1F
•
R_TRACKA + R_TRACKB
R_TRACKA
V_OUT1
V_OUT2
R_TRACKB
R_TRACKA
R1B LTC3836
R2B
V_FB1 V_FB2
R1A
R2A
TRACK/SS2
3836 F07a
Figure 7a. Using the TRACK/SS Pin
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
(7b) Coincident Tracking
TIME
3836 F07b_c
(7c) Ratiometric Tracking
Figures 7b and 7c. Two Different Modes of Output Voltage Tracking
3836fb
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