LTC1703_15 Datasheet, PDF (13/36 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID

LTC1703
APPLICATIO S I FOR ATIO
is limited to 10%. The maximum duty cycle limit increases
linearly between 1V and 2.5V, reaching its final value of
90% when RUN/SS is above 2.5V. 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 0.5V below V_CC, the MIN feedback comparator
is enabled, and the LTC1703 is in full operation.
CURRENT LIMIT
The LTC1703 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 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 LTC1703 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 LTC1703
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 LTC1703 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 g_m
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 output 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 LTC1703 to experience brief
overload conditions without affecting the output voltage
regulation. The delay also acts as a pole in the current limit
loop to enhance loop stability. Larger overloads cause the
soft-start capacitor to pull down quickly, protecting the
output components from damage. The current limit g_m
amplifier includes a clamp to prevent it from pulling RUN/
SS below 0.5V and shutting off the device.
Power MOSFET R_DS(ON) varies from MOSFET to MOSFET,
limiting the accuracy obtainable from the LTC1703 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 LTC1703 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 value for R_IMAX.
DISCONTINUOUS/Burst Mode OPERATION
Theory of operation
The LTC1703 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 LTC1703, 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
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