LTC3615-1_15 Datasheet, PDF (14/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615/LTC3615-1
Operation
Low Supply Operation
The LTC3615 is designed to operate down to an input supply
voltage of 2.25V. An important consideration at low input
supply voltages is that the R_DS(ON) of the P-channel and
N-channel power switches increases by 50% compared to
5V. The user should calculate the power dissipation when
the LTC3615 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in current mode
constant-frequency architectures by preventing subhar-
monic oscillations at duty cycles greater than 50%. The
LTC3615 implements slope compensation by adding a
compensation ramp to the inductor current signal.
Short-Circuit Protection
The peak inductor current at which the current comparator
shuts off the top power switch is controlled by the voltage
on the ITH pin.
If the output current increases, the error amplifier raises
the ITH pin voltage until the average inductor current
matches the new load current. In normal operation, the
LTC3615 clamps the maximum ITH pin voltage at ap-
proximately 1.05V which corresponds to about 5A peak
inductor current.
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. The
LTC3615 uses two techniques to prevent current runaway
from occurring:
1. If the output voltage drops below 50% of its nominal
value, the clamp voltage at pin ITH is lowered, causing
the maximum peak inductor current to lower gradu-
ally with the output voltage. When the output voltage
reaches 0V, the clamp voltage at the ITH pin drops to
40% of the clamp voltage during normal operation. The
short-circuit peak inductor current is determined by the
minimum on-time of the LTC3615, the input voltage
and the inductor value. This foldback behavior helps
in limiting the peak inductor current when the output
is shorted to ground. It is disabled during internal or
external soft-start and tracking up/down operation (see
the Applications Information section).
2. If the inductor current of the bottom MOSFET increases
beyond 6A typical, the top power MOSFET will be held
off and switching cycles will be skipped until the induc-
tor current reduces.
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