LTC1873_15 Datasheet, PDF (13/32 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID

LTC1873
APPLICATIO S I FOR ATIO
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 LTC1873 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 LTC1873 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_IMAX.
DISCONTINUOUS/Burst Mode OPERATION
Theory of operation
The LTC1873 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 LTC1873, 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
LTC1873 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 LTC1873) and the output
ripple voltage and frequency remain constant at all loads,
easing filtering requirements. Circuits that take advantage
of this behavior can force the LTC1873 to operate in
continuous mode at all loads by tying the FCB (Force
Continuous Bar) pin to ground.
Discontinuous Mode
To minimize the efficiency loss due to reverse current flow
at light loads, the LTC1873 switches to a second mode of
operation: discontinuous mode (Figure 5b). In discontinu-
ous mode, the LTC1873 detects when the inductor current
approaches zero and turns off QB for the remainder of the
switch cycle. During this time, the voltage at the SW pin
will float about V_OUT, the voltage across the inductor will
be zero, and the inductor current remains zero until the
next switching cycle begins and QT turns on again. This
prevents current from flowing backwards in QB, eliminat-
ing that power loss term. It also reduces the ripple current
in the inductor as the output current approaches zero.
The LTC1873 detects that the inductor current has reached
zero by monitoring the voltage at the SW pin while QB is
on. Since QB acts like a resistor, SW should ideally be right
at 0V when the inductor current reaches zero. In reality, the
SW node will ring to some degree immediately after it is
switched to ground by QB, causing some uncertainty as to
the actual moment the average current in QB goes to zero.
The LTC1873 minimizes this effect by ignoring the SW
node for a fixed 50ns after QB turns on when the ringing
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