LTC3731H Datasheet, PDF (17/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
APPLICATIO S I FOR ATIO
able 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 cur-
rent, 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:
R_SENSE
=
N
50mV
I_MAX
The IC works well with values of R_SENSE from 0.002Ω to
0.02Ω.
V_CC Decoupling
The V_CC pin supplies power not only to the internal circuits
of the controller but also to the top and bottom gate
drivers on the LTC3731CUH and therefore must be by-
passed very carefully to ground with a ceramic capacitor,
type X7R or X5R (depending upon the operating tempera-
ture 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 capaci-
tance of ALL of the MOSFETs being driven. Good bypass-
ing close to the IC is necessary to supply the high transient
currents required by the MOSFET gate drivers while keep-
ing 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 topside
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_OUT = 0.6V^{⎛⎝⎜}1+ _RR21^{⎞⎠⎟}
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
current 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.
3731hf
17