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LTC1705 Datasheet(PDF) 15 Page - Linear Technology

Part No. LTC1705
Description  Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1705 Datasheet(HTML) 15 Page - Linear Technology

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LTC1705
15
APPLICATIO S I FOR ATIO
The RUN/SS pin shuts down the LTC1705 if it falls below
0.5V (Figure 4). Between 0.5V and about 1V, the LTC1705
wakes up and the duty cycle is kept to a miminum. As the
potential at RUN/SS increases, the duty cycle increases
linearly between 1V and 2V, reaching its final value of 90%
when RUN/SS is above 2V. Somewhere before this point,
the feedback amplifier will assume control of the loop and
the output will come into regulation. When RUN/SS rises
to 1V below VCC , the MIN feedback comparator is enabled
and the LTC1705 is in full operation.
includes a trimmed 10
µA pull-up, enabling the user to set
the voltage at IMAX with a single resistor, RIMAX, to ground.
The LTC1705 compares the two inputs and begins limiting
the output current when the magnitude of the negative
voltage at the SW pin is greater than the voltage at IMAX .
When the load current increases abruptly, the voltage
feedback loop forces the duty cycle to increase rapidly and
the on-time of QB will be small momentarily. The RDS(ON)
of QB must be low enough to ensure that the SW node is
pulled low within the QB on-time for proper current
sensing.
The current limit detector is connected to an internal gm
amplifier that pulls a current from the RUN/SS pin propor-
tional to the difference in voltage magnitudes between the
SW and IMAX pins. The maximum value of this current is
250
µA (typically). It begins to discharge the soft-start
capacitor at RUN/SS, reducing the duty cycle and control-
ling the output voltage until the current drops below the
limit. The soft-start capacitor needs to move a fair amount
before it has any effect on the duty cycle, adding a delay
until the current limit takes effect (Figure 4). This allows
the LTC1705 to experience brief overload conditions with-
out affecting the output voltage regulation. The delay also
acts as a pole in the current limit loop to enhance loop
stability. While the soft-start capacitor is being discharged,
the top MOSFET must withstand the high power dissipa-
tion due to the high current especially if the regulator is
powered by a high current supply. Larger overloads cause
the soft-start capacitor to pull down quickly, protecting the
output components from damage. The current limit gm
amplifier includes a clamp to prevent it from pulling RUN/
SS below 0.5V and shutting off the LTC1705.
Power MOSFET RDS(ON) varies from MOSFET to MOSFET,
limiting the accuracy obtainable from the LTC1705 current
limit loop. Additionally, ringing on the SW node due to
parasitics can add to the apparent current, causing the
loop to engage early. The LTC1705 current limit is de-
signed primarily as a disaster prevention, “no blow up”
circuit and is not useful as a precision current regulator. It
should typically be set around 50% above the maximum
expected normal output current to prevent component
tolerances from encroaching on the normal current range.
See the Current Limit Programming section for advice on
choosing a valve for RIMAX .
Figure 4. Soft-Start Operation in Start Up and Current Limit
0V
VOUT
5V
4V
2V
1V
0.5V
MINIMUM DUTY CYCLE
0V
1705 F04
POWER-DOWN MODE
LTC1705 ENABLE
MIN
COMPARATOR
ENABLE
RUN/SS CONTROLS
DUTY CYCLE
COMP CONTROLS
DUTY CYCLE
START UP
NORMAL
OPERATION
CURRENT
LIMIT
Current Limit
The LTC1705 includes an onboard current limit circuit that
limits the maximum output current to a user-programmed
level. It works by sensing the voltage drop across QB
during the time that QB is on and comparing that voltage
to a user-programmed voltage at IMAX. Since QB looks like
a low value resistor during its on-time, the voltage drop
across it is proportional to the current flowing in it. In a
buck converter, the average current in the inductor is equal
to the output current. This current also flows through QB
during its on-time. Thus, by watching the voltage across
QB, the LTC1705 can monitor the output current.
Any time QB is on and the current flowing to the output is
reasonably large, the SW node at the drain of QB will be
somewhat negative with respect to PGND. The LTC1705
senses this voltage, inverts it and compares the sensed
voltage with a positive voltage at the IMAX pin. The IMAX pin


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