LTC3622_15 Datasheet, PDF (11/24 Pages) Linear Technology – 17V, Dual 1A Synchronous Step-Down Regulator with Ultralow Quiescent Current

LTC3622/
LTC3622-2/LTC3622-23/5
Operation
V_IN Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage events, the LTC3622 constantly
monitors the V_IN1 and V_IN2 pins for an overvoltage condi-
tion. When V_IN1 or V_IN2 rise above 18.5V, both regulators
suspend operation by shutting off both power MOSFETs.
Once V_IN drops below 18.2V, the regulator immediately
resumes normal operation. The regulators execute soft-
start when exiting an overvoltage condition.
Low Supply Operation
The LTC3622 incorporates undervoltage lockout circuits
which shut down the part when the input voltages drop
below 2.5V. As the input voltages rise slightly above the
undervoltage threshold, the switchers will begin basic op-
eration. However, the R_DS(ON) of the top and bottom switch
of each channel will be slightly higher than that specified
in the electrical characteristics due to lack of gate drive.
Refer to graph of R_DS(ON) versus V_IN for more details.
Phase Selection
The two channels of LTC3622 can operate in phase, 180°
out-of-phase (anti-phase) depending on the state of PHASE
pin- low, or high, respectively. Anti-phase generally re-
duces input voltage and current ripple. Crosstalk between
switch nodes SW1, SW2 and components or sensitive
lines connected to FBx, can sometimes cause unstable
switching waveforms and unexpectedly large input and
output voltage ripple.
The situation improves if rising and falling edges of the
switch nodes are timed carefully not to coincide. Depending
on the duty cycle of the two channels, choose the phase
difference between the channels to keep edges as far away
from each other as possible.
Crosstalk can generally be avoided by carefully choosing
the phase shift such that the SW edges do not coincide.
However, there are often situations where this is unavoid-
able, such as when both channels are operating at near
50% duty cycle. In such cases, the optimized phase shift
can be set by modulating the duty cycle of external clock
on the MODE/SYNC pin (channel 1 edge synced to rising
edge of external clock, channel 2 edge synced to falling
edge of external clock), while keeping the PHASE pin volt-
age high. Figure 1 shows a 90° phase shifting between
two channels. Table 1 shows the phase selection by the
PHASE pin.
EXTERNAL
CLOCK
SW1
SW2
500ns/DIV
3622 F01
Figure 1. 90° Phase Shift Set by External Clock
Table 1. Phase Selection
NO EXTERNAL CLK EXTERNAL CLK
PHASE = 0
0° Phase Shift
0° Phase Shift
PHASE = INTV_CC 180° Phase Shift
Phase Shift Determined by
Clock Edges
Soft-Start
The LTC3622 has a 500µs soft-start ramp for each channel
when enabled. During soft-start operation, the switchers
operate in pulse-skipping mode.
For more information www.linear.com/LTC3622
3622fa
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