LTC3892-1_15 Datasheet, PDF (21/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
RUN Pins
The LTC3892/LTC3892-1 is enabled using the RUN1 and
RUN2 pins. The RUN pins have a rising threshold of 1.275V
with 75mV of hysteresis. Pulling a RUN pin below 1.2V
shuts down the main control loop for that channel. Pulling
both RUN pins below 0.7V disables the controllers and
most internal circuits, including the DRV_CC and INTV_CC
LDOs. In this state, the LTC3892/LTC3892-1 draws only
3.6µA of quiescent current.
Releasing a RUN pin allows a small 150nA internal current
to pull up the pin to enable that controller. Because of
condensation or other small board leakage pulling the pin
down, it is recommended the RUN pins be externally pulled
up or driven directly by logic. Each RUN pin can tolerate
up to 65V (absolute maximum), so it can be conveniently
tied to V_IN in always-on applications where one or more
controllers are enabled continuously and never shut down.
The RUN pins can be implemented as a UVLO by con-
necting them to the output of an external resistor divider
network off V_IN, as shown in Figure 4.
1/2 LTC3892/
LTC3892-1
RUN
V_IN
R_B
R_A
3892 F04
Figure 4. Using the RUN Pins as a UVLO
The rising and falling UVLO thresholds are calculated using
the RUN pin thresholds and pull-up current:
V_UVLO(RISING)
=
1.275V


1+
R_B
R_A


–
150nA
• R_B
Tracking and Soft-Start (TRACK/SS1, TRACK/SS2 Pins)
The start-up of each V_OUT is controlled by the voltage on
the TRACK/SS pin (TRACK/SS1 for channel 1, TRACK/SS2
for channel 2). When the voltage on the TRACK/SS pin
is less than the internal 0.8V reference, the LTC3892/
LTC3892-1 regulates the V_FB pin voltage to the voltage
on the TRACK/SS pin instead of the internal reference.
The TRACK/SS pin can be used to program an external
soft-start function or to allow V_OUT to track another sup-
ply during start-up.
Soft-start is enabled by simply connecting a capacitor
from the TRACK/SS pin to ground, as shown in Figure 5.
An internal 10μA current source charges the capacitor,
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3892/LTC3892-1 will regulate its feedback volt-
age (and hence V_OUT) according to the voltage on the
TRACK/SS pin, allowing V_OUT to rise smoothly from 0V
to its final regulated value. The total soft-start time will be
approximately:
t_SS
=
C_SS
•
0.8V
10µA
1/2 LTC3892/
LTC3892
TRACK/SS
C_SS
GND
38921 F06
Figure 5. Using the TRACK/SS Pin to Program Soft-Start
V_{UVLO(FALLING)}
=
1.20V


1+
R_B
R_A


–
150nA
• R_B
For more information www.linear.com/LTC3892
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