LTC3731H
17
3731Hfb
applicaTions inForMaTion
available in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POS-CAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturer for other specific recommendations.
R
SENSE
Selection for Output Current
Once the frequency and inductor have been chosen,
R
SENSE1
, R
SENSE2
, R
SENSE3
are determined based on the
required peak inductor current. The current comparator
has a typical maximum threshold of 75mV/R
SENSE
and
an input common mode range of SGND to (1.1) • V
CC
.
The current comparator threshold sets the peak inductor
current, yielding a maximum average output current I
MAX
equal to the peak value less half the peak-to-peak ripple
current, ∆I
L
.
Allowing a margin for variations in the IC and external
component values yields:
The IC works well with values of R
SENSE
from 0.002Ω to
0.02Ω.
V
CC
Decoupling
The V
CC
and V
DR
pins supply power to the internal cir-
cuits of the controller and to the top and bottom gate
drivers. Therefore, they must be bypassed very carefully
to ground with ceramic capacitors, type X7R or X5R (de-
pending upon the operating temperature environment)
of at least 1µF immediately next to the IC and preferably
an additional 10µF placed very close to the IC due to the
extremely high instantaneous currents involved. The total
capacitance, taking into account the voltage coefficient
of ceramic capacitors, should be 100 times as large as
the total combined gate charge capacitance of ALL of the
MOSFETs being driven. Good bypassing close to the IC is
necessary to supply the high transient currents required
by the MOSFET gate drivers while keeping the 5V supply
quiet enough so as not to disturb the very small-signal
high bandwidth of the current comparators.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors, C
B
, connected to the
BOOST pins, supply the gate drive voltages for the top-
side MOSFETs. Capacitor C
B
in the Functional Diagram
is charged though diode D
B
from V
CC
when the SW pin
is low. When one of the topside MOSFETs turns on, the
driver places the C
B
voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns
on the topside switch. The switch node voltage, SW,
rises to V
IN
and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply
(V
BOOST
= V
CC
+ V
IN
). The value of the boost capacitor C
B
needs to be 30 to 100 times that of the total gate charge
capacitance of the topside MOSFET(s) as specified on the
manufacturer’s data sheet. The reverse breakdown of D
B
must be greater than V
IN(MAX)
.
The output voltage is set by an external resistive divider
according to the following formula:
V V
R
R
OUT
= +
0 6 1
1
2
.
The resistive divider is connected to the output as shown
in Figure 2.
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) ON/OFF, 2)
soft-start and 3) a defeatable short-circuit latch off timer.
Soft-start reduces the input power sources’ surge currents
by gradually increasing the controller’s current limit (pro-
portional to an internal buffered and clamped V
ITH
). The
latchoff timer prevents very short, extreme load transients
from tripping the overcurrent latch. A small pull-up cur-
rent (>5µA) supplied to the RUN/SS pin will prevent the
overcurrent latch from operating. A maximum pull-up cur-
rent of 200µA is allowed into the RUN/SS pin even though
the voltage at the pin may exceed the absolute maximum
rating for the pin. This is a result of the limited current
and the internal protection circuit on the pin. The following
explanation describes how this function operates.
