LTC3868IUH#TRPBF Linear Technology, LTC3868IUH#TRPBF Datasheet - Page 12

• Manufacturer Part Number: LTC3868IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3868IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 14 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

OPERATION
LTC3868
The delay time from when a short-circuit occurs until the
controller latches off can be calculated using the follow-
ing equation:
where V
on the SS pin at the time the short-circuit occurs. Normally
the SS pin voltage will have been pulled up to the INTV
voltage (5.1V) by the internal 1μA pull-up current.
Note that the two controllers on the LTC3868 have separate,
independent short-circuit latchoff circuits. Latchoff can be
overridden/defeated by connecting a resistor 150k or less
from the SS pin to INTV
pull-up current to overcome the 9μA pull-down current
present during a short-circuit. Note that this resistor also
shortens the soft-start period.
Foldback Current
On the other hand, when the output voltage falls to less
than 72% 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
12
COMMAND
LATCHOFF
LATCHOFF
CURRENT
VOLTAGE
OUTPUT
t
ENABLE
LATCH
SS PIN
0.8V
SS
0V
~ C
is the initial voltage (must be greater than 2V)
2V
SS
1μA
Figure 1. Latchoff Timing Diagram
SOFT-START INTERVAL
(V
INTV
SS
ARMING
CC
(Refer to the Functional Diagram)
– 1.5V)/9μA
CC
. This resistor provides enough
–9μA
t
LATCH
1.5V
1μA
3868 F01
CC
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 effi cient nature of
the current mode switching regulator. Foldback current
limiting is disabled during the soft-start interval (as long
as the V
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping or Forced Continuous Mode)
(PLLIN/MODE Pin)
The LTC3868 can be enabled to enter high effi ciency
Burst Mode operation, constant frequency pulse-skip-
ping 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
operation, tie the PLLIN/MODE pin to INTV
pulse-skipping mode, tie the PLLIN/MODE pin to a DC
voltage greater than 1.2V and less than INTV
When a controller is enabled for Burst Mode operation, the
minimum peak current in the inductor is set to approxi-
mately 30% of the maximum sense voltage even though
the voltage on the I
average inductor current is higher than the load current,
the error amplifi er EA will decrease the voltage on the I
pin. When the I
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 draws
only 170μA of quiescent current. If both channels are in
sleep mode, the LTC3868 draws only 300μA of quiescent
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
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.
FB
voltage is keeping up with the SS voltage).
TH
TH
pin is reconnected to the output of the
voltage drops below 0.425V, the internal
TH
pin indicates a lower value. If the
CC
CC
. To select
– 1.3V.
3868fd
TH