LTC1704/LTC1704B
14
1704bfa
APPLICATIO S I FOR ATIO
WUU
U
The delay also acts as a pole in the current limit loop to
enhance loop stability. Prolonged overload conditions will
allow the RUN/SS pin to reach a steady state, and the
output will remain at a reduced voltage until the overload
is removed. Under current limit condition, if the output
voltage is less than 10% of its normal value, the soft-start
capacitor will be forced low immediately and the LTC1704
will rerun a complete soft-start cycle. The soft-start ca-
pacitor must be selected such that during power-up the
current through QB will not exceed the current limit value.
Power MOSFET R
DS(ON)
varies from MOSFET to MOSFET,
limiting the accuracy obtainable from the LTC1704 current
limit loop. Additionally, ringing on the SW node due to
parasitics can add to the apparent current, causing the
loop to engage early. 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 R
DS(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 LTC1704 current limit
is designed primarily as a disaster prevention, “no blow-
up” circuit, and is not useful as a precision current regu-
lator. It should typically be set around 50% above the
maximum expected normal output current to prevent com-
ponent tolerances from encroaching on the normal cur-
rent range. See the Switching Supply Current Limit Pro-
gramming section for advice on choosing a valve for R
IMAX
.
BURST MODE OPERATION
Theory of Operation
The LTC1704 (non-B part) switcher supply has two modes
of operation. Under heavy loads, it operates as a fully
synchronous, continuous conduction switching regula-
tor. 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 operation 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 LTC1704, 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
LTC1704 remains in Continuous mode. See the Inductor
Selection section for a detailed description of ripple
current.
As the output load current decreases in Continuous mode,
the average current in the inductor will reach a point where
it drops below half the ripple current. At this point, the
current in the inductor will reverse during a portion of the
switching cycle, or begin to flow from the output back to
the input. This does not adversely affect regulation, but
does cause additional losses as a portion of the inductor
current flows back and forth through the resistive power
switches, giving away a little more power each time and
lowering the efficiency. There are some benefits to allow-
ing this reverse current flow: the circuit will maintain
regulation even if the load current drops below zero (the
load supplies current to the LTC1704) and the output
ripple voltage and frequency remain constant at all loads,
easing filtering requirements.
Besides the reverse current loss, the LTC1704 drivers are
still switching QT and QB on and off once a cycle. Each time
an external MOSFET is turned on, the internal driver must
charge its gate to PV
CC
. Each time it is turned off, that
charge is lost to ground. At the high switching frequency
that the LTC1704 operates, the charge lost to the gates can
add up to tens of milliamps from PV
CC
. As the load current
continues to drop, this quickly becomes the dominant
power loss term, reducing efficiency once again.
To minimize the efficiency loss due to switching loss and
reverse current flow at light loads, the LTC1704 (non-B
part) switches to a second mode of operation: Burst Mode
operation (Figure 5b). In Burst Mode operation, the
LTC1704 detects when the inductor current approaches
zero and turns off both drivers. During this time, the
voltage at the SW pin will float around V
OUTSW
, the voltage
(For Non-B Parts Only)
