LTC3707 Datasheet, PDF (21/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
but less than in normal operation. The short-circuit ripple
current is determined by the minimum on-time t_ON(MIN)
of the LTC3707 (less than 200ns), the input voltage and
inductor value:
ΔI_L(SC) = t_ON(MIN) (V_IN/L)
The resulting short-circuit current is:
I_SC
=
25mV
R_SENSE
+
1
2
ΔI_L(SC)
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
A comparator monitors the output for overvoltage con-
ditions. The comparator (0V) detects overvoltage faults
greater than 7.5% above the nominal output voltage. When
this condition is sensed, the top MOSFET is turned off and
the bottom MOSFET is turned on until the overvoltage
condition is cleared. The output of this comparator is
only latched by the overvoltage condition itself and will
therefore allow a switching regulator system having a poor
PC layout to function while the design is being debugged.
The bottom MOSFET remains on continuously for as long
as the 0V condition persists; if V_OUT returns to a safe level,
normal operation automatically resumes. A shorted top
MOSFET will result in a high current condition which will
open the system fuse. The switching regulator will regulate
properly with a leaky top MOSFET by altering the duty
cycle to accommodate the leakage.
The Standby Mode (STBYMD) Pin Function
The Standby Mode (STBYMD) pin provides several choices
for start-up and standby operational modes. If the pin is
pulled to ground, the RUN/SS pins for both controllers
are internally pulled to ground, preventing start-up and
thereby providing a single control pin for turning off both
controllers at once. If the pin is left open or decoupled with
a capacitor to ground, the RUN/SS pins are each internally
provided with a starting current enabling external control
for turning on each controller independently. If the pin is
provided with a current of >3μA at a voltage greater than
2V, both internal linear regulators (INTV_CC and 3.3V) will
be on even when both controllers are shut down. In this
mode, the onboard 3.3V and 5V linear regulators can
provide power to keep-alive functions such as a keyboard
controller. This pin can also be used as a latching “on”
and/or latching “off” power switch if so designed.
Frequency of Operation
The LTC3707 has an internal voltage controlled oscillator.
The frequency of this oscillator can be varied over a 2 to 1
range. The pin is internally self-biased at 1.19V, resulting
in a free-running frequency of approximately 220kHz. The
FREQSET pin can be grounded to lower this frequency to
approximately 140kHz or tied to the INTV_CC pin to yield
approximately 310kHz. The FREQSET pin may be driven
with a voltage from 0 to INTV_CC to fix or modulate the
oscillator frequency as shown in Figure 5.
Minimum On-Time Considerations
Minimum on-time t_ON(MIN) is the smallest time duration
that the LTC3707 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty
cycle applications may approach this minimum on-time
limit and care should be taken to ensure that
t_ON(MIN)
<
V_OUT
V_IN(f)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3707 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The typical tested minimum on-time of the LTC3707 is
180ns under an ideal condition without switching noise.
However, the minimum on-time can be affected by PCB
switching noise in the voltage and current loops. With
reasonably good PCB layout, minimum 30% inductor
current ripple and about 15mV sensing ripple voltage,
300ns minimum on-time is a conservative number to
start with.
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