LTC3714EG#TR Linear Technology, LTC3714EG#TR Datasheet - Page 8

IC STP-DWN CNTRLR W/OPAMP 28SSOP

LTC3714EG#TR

Manufacturer Part Number
LTC3714EG#TR
Description
IC STP-DWN CNTRLR W/OPAMP 28SSOP
Manufacturer
Linear Technology
Datasheet

Specifications of LTC3714EG#TR

Applications
Controller, Intel Pentium®
Voltage - Input
4 ~ 36 V
Number Of Outputs
1
Voltage - Output
0.6 ~ 1.75 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
28-SSOP
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
Other names
LTC3714EGTR

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operaTion
LTC3417A
The LTC3417A uses a constant frequency, current mode
architecture. Both channels share the same clock frequency.
The PHASE pin sets whether the channels are running
in-phase or out of phase. The operating frequency is de-
termined by connecting the FREQ pin to V
operation or by connecting a resistor from FREQ to ground
for a frequency from 0.6MHz to 4MHz. To suit a variety of
applications, the SYNC/MODE pin allows the user to trade
off noise for efficiency.
The output voltages are set by external dividers returned
to the V
divided output voltage with a reference voltage of 0.8V and
adjusts the peak inductor current accordingly. Undervoltage
comparators will pull the PGOOD output low when either
output voltage is 6% below its targeted value.
Main Control Loop
For each regulator, during normal operation, the P-chan-
nel MOSFET power switch is turned on at the beginning
of a clock cycle when the V
ence voltage. The current into the inductor and the load
increases until the current limit is reached. The switch
turns off and energy stored in the inductor flows through
the bottom N-channel MOSFET switch into the load until
the next clock cycle.
The peak inductor current is controlled by the voltage
on the I
This amplifier compares the V
When the load current increases the V
slightly below the reference. This decrease causes the er-
ror amplifier to increase the I
inductor current matches the new load current.
The main control loop is shut down by pulling the RUN pin
to ground. A digital soft-start is enabled after shutdown,
which will slowly ramp the peak inductor current up over
1024 clock cycles.
Low Current Operation
Three modes are available to control the operation of
the LTC3417A at low currents. Each of the three modes
automatically switch from continuous operation to the
selected mode when the load current is low.
8
FB1
TH
pin, which is the output of the error amplifier.
and V
FB2
pins. An error amplifier compares the
FB
TH
FB
voltage is below the refer-
pin to the 0.8V reference.
voltage until the average
FB
voltage decreases
IN
for 1.5MHz
To optimize efficiency, Burst Mode operation can be
selected. When the load is relatively light, the LTC3417A
automatically switches into Burst Mode operation in which
the PMOS switches operate intermittently based on load
demand. By running cycles periodically, the switching
losses, which are dominated by the gate charge losses
of the power MOSFETs, are minimized. The main control
loop is interrupted when the output voltage reaches the
desired regulated value. The hysteresis voltage comparator
trips when I
reducing the power. The output capacitor and the induc-
tor supply the power to the load until I
turning on the switch and the main control loop which
starts another cycle.
For lower output voltage ripple at low currents, pulse
skipping mode can be used. In this mode, the LTC3417A
continues to switch at constant frequency down to very
low currents, where it will begin skipping pulses used to
control the power MOSFETs.
Finally, in forced continuous mode, the inductor current is
constantly cycled creating a fixed output voltage ripple at all
output current levels. This feature is desirable in telecom-
munications since the noise is a constant frequency and is
thus easy to filter out. Another advantage of this mode is
that the regulator is capable of both sourcing current into
a load and sinking some current from the output.
The mode selection for the LTC3417A is set using the
SYNC/MODE pin. The SYNC/MODE pin sets the mode for
both the1A and the 1.5A step-down DC/DC converters.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases to 100%. In this dropout
condition, the PMOS switch is turned on continuously with
the output voltage being equal to the input voltage minus
the voltage drops across the internal P-channel MOSFET
and inductor.
Low Supply Operation
The LTC3417A incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.07V to prevent unstable operation.
TH
is below 0.24V, shutting off the switch and
TH
exceeds 0.31V,
3417afc

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