LTC1704EGN Datasheet, PDF (13/28 Pages) Linear Technology – 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller

LTC1704/LTC1704B
APPLICATIO S I FOR ATIO
low, indicating that the output is out of regulation. For
PGOOD to go high, both the outputs must be in regulation
for more than 20µs. PGOOD remains active during soft-
start and current limit. Upon power-up, PGOOD is forced
low. As soon as the RUN/SS and REGILM pins rise above
the shutdown thresholds, the two pairs of power good
comparators take over and control the transistor MPG
directly. The 1µs and 20µs delay ensures that short output
transient glitches that are successfully “caught” by the
power good comparators don’t cause momentary glitches
at the PGOOD pin.
Shutdown/Soft-Start
The RUN/SS pin performs two functions: when pulled to
ground, it shuts down the switcher drivers, and acts as a
conventional soft-start pin, enforcing a maximum duty
cycle limit proportional to the voltage at RUN/SS. An
internal 3µA current source pull-up is connected to the
RUN/SS pin, allowing a soft-start ramp to be generated
with a single external capacitor to ground. The 3µA current
source is active even when the LTC1704 is shut down,
ensuring the device will start when any external pull-down
at RUN/SS is released.
The RUN/SS pin shuts down the switcher drivers when it
falls below 0.5V (Figure 4). Between 0.5V and about 1V,
the LTC1704 wakes up and the duty cycle is kept to
minimum. As the potential at RUN/SS goes higher, the
duty cycle increases linearly between 1V and 2V, reaching
its final value of 90% when RUN/SS is above 2V. Some-
where before this point, the feedback amplifier will as-
sume control of the loop and the output will come into
regulation. When RUN/SS rises to 1V below V_CC , the MIN
feedback comparator is enabled, and the LTC1704 voltage
feedback loop is in full operation.
Switcher Supply Current Limit
The LTC1704 switcher supply includes an onboard cur-
rent 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 compar-
ing that voltage to a user-programmed voltage at I_MAX.
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 LTC1704 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 LTC1704
senses this voltage and inverts it to allow it to compare the
sensed voltage with a positive voltage at the I_MAX pin. The
I_MAX pin includes a trimmed 10µA pull-up, enabling the
user to set the voltage at I_MAX with a single resistor, R_IMAX,
to ground. The LTC1704 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 I_MAX.
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 I_MAX pins. This current begins to discharge the
soft-start capacitor at RUN/SS, reducing the duty cycle
and controlling 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 LTC1704 to experience brief overload
conditions without affecting the output voltage regulation.
V_OUT
0V
5V
4V
2V
1V
0.5V
0V
START-UP
CURRENT
LIMIT
NORMAL
OPERATION
COMP CONTROLS
MIN
DUTY CYCLE COMPARATOR
ENABLE
RUN/SS CONTROLS
DUTY CYCLE
MINIMUM DUTY CYCLE
DRIVER DISABLE MODE
LTC1704 ENABLE
HARD
CURRENT
LIMIT
1704 F04
Figure 4. Soft-Start Operation in Start Up and Current Limit
1704bfa
13