LTC3118_15 Datasheet, PDF (17/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Operation
The maximum load current capability in PWM mode curves
in the Typical Performance Characteristics section show
the relationship of input voltage and the ability to deliver
load current at V_OUT = 5V and 12V. When the input voltage
is a volt or more above V_OUT in buck mode, the LTC3118
is capable of providing more than 2A of load current. In
boost mode, the output current capability is further reduced
by the boost ratio or duty cycle (D) as described in the
preceding equation.
Overload Current Limit and Reverse Current
Comparators
The internal current sense waveforms are used by the
peak overload current (I_PEAK) and reverse current (I_REV)
comparators. The I_PEAK current comparator monitors
I_SENSE and interrupts normal PWM operation if the induc-
tor current level exceeds its maximum internal threshold.
This threshold is approximately 60% above the maximum
average current level of the current control loop. If the
internal current sense waveform rises above this level, the
LTC3118 will disconnect the inductor from V_IN by shut-
ting off switch A1 (or A2) to prevent higher current in the
inductor. The I_PEAK circuitry is reset by the oscillator clock
at the end of each switching cycle. In the event that the
overload comparator is tripped as the result of an output
short-circuit condition, where V_OUT is discharged below
approximately 1V, the LTC3118 will initiate a soft-start
event keeping the on-chip power dissipation to low levels.
Once the short circuit is removed, the LTC3118 will restart
in the normal fashion. If the average current loop is able
to prevent inductor current from reaching I_PEAK during a
short-circuit event, soft-start will not be initiated, but the
maximum current capability of the current loop will be
reduced by 40% to reduce power dissipation.
The LTC3118 contains a reverse current comparator set to
a nominal value of –200mA. If the internal current sense
waveform transitions below the internally set reverse cur-
rent threshold, the LTC3118 will disconnect the inductor
from V_OUT by shutting off switch D, to prevent rapid dis-
charge of the output capacitor. The I_REV circuitry is reset
by the oscillator clock at the end of the switching cycle.
Burst Mode Operation
When the MODE pin is held low, the LTC3118 is configured
for automatic Burst Mode operation. As a result, the buck-
boost DC/DC converter will operate with normal continuous
PWM switching above a predetermined average inductor
current and will automatically transition to power saving
Burst Mode operation below this level. Refer to the Typical
Performance Characteristics section of this data sheet to
determine the Burst Mode transition threshold for various
combinations of V_IN and V_OUT.
With MODE held low at light output loads, the LTC3118
will go into a standby or sleep state when the output volt-
age achieves its nominal regulation level. The sleep state
halts PWM switching and powers down all nonessential
functions of the IC, significantly reducing the quiescent
current of the LTC3118. This greatly improves overall
power conversion efficiency when the output load is
light. Since the converter does not operate in sleep, the
output voltage will slowly decay at a rate determined by
the output load resistance and the output capacitor value.
When the output voltage has decayed by a small amount,
the LTC3118 will wake up and resume normal PWM
switching operation until the voltage on V_OUT is restored
to the previous level. If the load is very light, the LTC3118
may only need to switch for a few cycles to restore V_OUT,
and may sleep for extended periods of time, significantly
improving conversion efficiency.
For more information www.linear.com/LTC3118
3118f
17