LTC1702ACGN#TR-SMI Linear Technology, LTC1702ACGN#TR-SMI Datasheet - Page 13

• Manufacturer Part Number: LTC1702ACGN#TR-SMI
• Manufacturer: Linear Technology

Specifications of LTC1702ACGN#TR-SMI

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIONS
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 LTC1702A
senses this voltage and inverts it to allow it to compare the
sensed voltage with a positive voltage at the I
I
user to set the voltage at I
to ground. The LTC1702A compares the two inputs and
limits the output current when the magnitude of the
negative voltage at the SW pin is greater than the voltage
at I
The LTC1702A current limit detector connects to an inter-
nal circuit that discharges the soft-start capacitor quickly
if activated. The soft-start sink current depends on the
overdrive presented to the current limit detector. If the
regulator output is short circuited, the soft-start sink
current is typically 1mA. With a soft-start capacitor less
than 0.01µF, the current-limit detector compensation is
slightly under damped. With an instantaneous short-
circuit, the current-limit detector fires and the soft-start
capacitor rapidly discharges to ground. Depending on the
current limit behavior of the regulator powering the
LTC1702A, current in the switch inductor of the shorted
output can exceed 100A before the soft-start capacitor is
discharged. This high input current surge also pulls down
the input voltage to the LTC1702A and all other circuits
connected to the LTC1702A input. After the soft-start
capacitor is discharged, the output is turned off and the
LTC1702A begins a new soft-start cycle. If the over current
condition persists, the current limit detector fires again
and the cycle repeats. With a soft-start capacitor greater
than 0.01µF, the current limit detector compensation is
slightly over damped. With an instantaneous short-circuit
condition, the soft-start capacitor is again quickly dis-
charged. However, the SS pin does not pull to ground but
only discharges until the current limit loop is in regulation.
Short-circuit current is limited to the programmed current
limit level. In this scenario, the LTC1702A regulates in
current limit and does not rerun soft-start cycles. There-
fore, the user must balance the trade off between soft-start
time required for the system versus desired current limit
behavior. Consult the Current Limit Programming section
for more information.
MAX
MAX
pin includes a trimmed 10µA pull-up, enabling the
.
U
MAX
INFORMATION
U
with a single resistor, R
W
MAX
U
pin. The
IMAX
,
Note that even brief overcurrent excursions will fire the
current limit circuit, quickly removing power to the load. If
the ability to withstand larger overcurrent surges without
tripping off is desired, consider using the pin-compatible
LTC1702, which provides this capability in exchange for
increased stress on the power MOSFETs.
Power MOSFET R
limiting the accuracy obtainable from the LTC1702A cur-
rent limit loop. Additionally, ringing on the SW node due to
parasitics can add to the apparent current, causing the
loop to engage early. The LTC1702A current limit is
designed 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 R
DISCONTINUOUS/Burst Mode OPERATION
Theory of operation
The LTC1702A switching logic has three modes of opera-
tion. Under heavy loads, it operates as a fully synchro-
nous, continuous conduction switching regulator. In this
mode of operation (“continuous” mode), the current in the
inductor flows in the positive direction (toward the output)
during the entire switching cycle, constantly supplying
current to the load. In this mode, the synchronous switch
(QB) is on whenever QT is off, so the current always flows
through a low impedance switch, minimizing voltage drop
and power loss. This is the most efficient mode of opera-
tion at heavy loads, where the resistive losses in the power
devices are the dominant loss term.
Continuous mode works efficiently when the load current
is greater than half of the ripple current in the inductor. In
a buck converter like the LTC1702A, the average current in
the inductor (averaged over one switching cycle) is equal
to the load current. The ripple current is the difference
between the maximum and the minimum current during a
switching cycle (see Figure 5a). The ripple current
depends on inductor value, clock frequency and output
voltage, but is constant regardless of load as long as the
DS(ON)
IMAX
varies from MOSFET to MOSFET,
.
LTC1702A
13
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