LTC3868-1 Datasheet, PDF (12/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3868-1
Operation (Refer to the Functional Diagram)
INTV_CC
SS VOLTAGE
2V
1.5V
0.8V
LATCHOFF
COMMAND
0V
SS PIN
CURRENT
1µA
1µA
–9µA
OUTPUT
VOLTAGE
LATCHOFF
ENABLE
ARMING
SOFT-START INTERVAL
t_LATCH
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Figure 1. Latchoff Timing Diagram
The delay time from when a short-circuit occurs until the
controller latches off can be calculated using the follow-
ing equation
t_LATCH ~ C_SS (V_SS – 1.5V)/9µA
where V_SS is the initial voltage (must be greater than 2V)
on the SS pin at the time the short-circuit occurs. Normally
the SS pin voltage will have been pulled up to the INTV_CC
voltage (5.1V) by the internal 1µA pull-up current.
Note that the two controllers on the LTC3868-1 have sepa-
rate, independent short-circuit latchoff circuits. Latchoff
can be overridden/defeated by connecting a resistor 150k
or less from the SS pin to INTV_CC. This resistor provides
enough pull-up current to overcome the 9µA pull-down
current present during a short-circuit. Note that this resis-
tor also shortens the soft-start period.
Foldback Current
On the other hand, when the output voltage falls to less
than 70% of its nominal level, foldback current limiting
is also activated, progressively lowering the peak current
limit in proportion to the severity of the overcurrent or
short-circuit condition. Even if a short-circuit is present
and the short-circuit latchoff is not yet enabled (when
SS voltage has not yet reached 2V), a safe, low output
current is provided due to internal current foldback and
actual power wasted is low due to the efficient nature of
the current mode switching regulator. Foldback current
limiting is disabled during the soft-start interval (as long
as the V_FB voltage is keeping up with the SS voltage).
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3868-1 can be enabled to enter high efficiency
Burst Mode operation, constant frequency pulse-skipping
mode, or forced continuous conduction mode at low load
currents. To select Burst Mode operation, tie the PLLIN/
MODE pin to ground. To select forced continuous opera-
tion, tie the PLLIN/MODE pin to INTV_CC. To select pulse-
skipping mode, tie the PLLIN/MODE pin to a DC voltage
greater than 1.2V and less than INTV_CC – 1.3V.
When a controller is enabled for Burst Mode operation,
the minimum peak current in the inductor is set to ap-
proximately 30% of the maximum sense voltage even
though the voltage on the I_TH pin indicates a lower value.
If the average inductor current is higher than the load
current, the error amplifier, EA, will decrease the voltage
on the I_TH pin. When the I_TH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current. If one channel is shut down
and the other channel is in sleep mode, the LTC3868-1
draws only 170µA of quiescent current. If both channels
are in sleep mode, the LTC3868-1 draws only 300µA of qui-
escent current. In sleep mode, the load current is supplied
by the output capacitor. As the output voltage decreases,
the EA’s output begins to rise. When the output voltage
drops enough, the I_TH pin is reconnected to the output
of the EA, the sleep signal goes low, and the controller
resumes normal operation by turning on the top external
MOSFET on the next cycle of the internal oscillator.
When a controller is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
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