LTC3124_15 Datasheet, PDF (13/28 Pages) Linear Technology – 15V, 5A 2-Phase Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3124
OPERATION
Boost Anti-Ringing Control
Output Disconnect
When V_OUT ≥ V_IN + 2V, the anti-ringing circuitry connects a
resistor across each inductor to V_IN to damp high frequency
ringing on the SW pins during discontinuous current mode
operation. Although the ringing of the resonant circuits
formed by the inductors and C_SW(A/B) (capacitance on
the respective SW pins) is low energy, it can cause EMI
radiation if not damped.
V_CC Regulator
An internal low dropout regulator generates the 4.25V
(nominal) V_CC rail from V_IN or V_OUT, depending upon
operating conditions. V_CC is supplied from V_IN if V_IN is
initially greater than approximately 3.5V, with V_IN continuing
to supply V_CC down to approximately 3V; otherwise the
greater of V_IN and V_OUT supplies V_CC. The V_CC rail powers
the internal control circuitry and power MOSFET gate drivers
of the LTC3124. The V_CC regulator is disabled in shutdown
to reduce quiescent current and is enabled by forcing the
SD pin above its input high threshold. A 4.7µF or larger
capacitor must be connected between V_CC and SGND.
The LTC3124’s output disconnect feature eliminates body
diode conduction of the internal P-channel MOSFET recti-
fiers. This feature allows for V_OUT to discharge to 0V during
shutdown, and draw no current from the input source.
Inrush current will also be limited at turn-on, minimizing
surge currents seen by the input supply. Note that to obtain
the advantages of output disconnect, there must not be
an external Schottky diode connected between SWA, SWB
and V_OUT. The output disconnect feature also allows V_OUT
to be pulled high, without backfeeding the power source
connected to V_IN.
V_IN > V_OUT Operation
The LTC3124 step-up converter will maintain voltage
regulation even when the input voltage is above the desired
output voltage. Note that operating in this mode will exhibit
lower efficiency and a reduced output current capability.
Refer to the Typical Performance Characteristics for details.
Burst Mode OPERATION
Overvoltage Lockout
An overvoltage condition occurs when V_OUT exceeds
approximately 16.5V. Switching is disabled and the in-
ternal soft-start ramp is reset. Once V_OUT drops below
approximately 16V a soft-start is initiated and switching
is allowed to resume. If the boost converter output is
lightly loaded such that the time constant of the output
capacitance, C_OUT, and the output load resistance, R_OUT
is near or greater than the soft-start time of approximately
10ms, the soft-start ramp may end before or soon after
switching resumes, defeating the inrush current limiting of
the closed-loop soft-start following an overvoltage event.
Short-Circuit Protection
The LTC3124 output disconnect feature allows output
short-circuit protection while maintaining a maximum set
current limit. To reduce power dissipation under overload
and short-circuit conditions, the peak switch current limits
are reduced to approximately 2A. Once V_OUT exceeds
approximately 1.5V, the current limits are reset to their
nominal values of 3.5A per phase.
When the PWM/SYNC pin is held low, the boost converter
operates in Burst Mode, independent of load current. This
mode of operation is typically commanded to improve
efficiency at light loads and reduce standby current at no
load. The output current (I_OUT) capability in Burst Mode
operation is significantly less than in PWM mode and
varies with V_IN and V_OUT, as shown in Figure 5. The logic
input thresholds for this pin are determined relative to V_CC
with a low being less than 10% of V_CC and a high being
greater than 90% of V_CC. The LTC3124 will operate in
fixed frequency PWM mode even if Burst Mode operation
is commanded during soft-start.
In Burst Mode operation, only Phase A of the LTC3124
is operational, while Phase B is disabled. The Phase A
inductor current is initially charged to approximately
700mA by turning on the N-channel MOSFET switch, at
which point the N-channel switch is turned off and the
P-channel synchronous switch is turned on, delivering
current to the output. When the inductor current discharges
to approximately zero, the cycle repeats. In Burst Mode
operation, energy is delivered to the output until the nominal
3124f
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