LTC3731 Datasheet, PDF (17/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
APPLICATIO S I FOR ATIO
series of surface mount special polymer capacitors avail-
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).
Differential Amplifier/Output Voltage Programming
The IC has a true remote voltage sense capability. The
sensing connections should be returned from the load,
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier rejects common mode signals capacitively or
inductively radiated into the feedback PC traces as well as
ground loop disturbances. The differential amplifier out-
put signal is divided down with an external resistive divider
and compared with the internal, precision 0.6V voltage
reference by the error amplifier.
The differential amplifier has a 0V to V_CC common mode
input range and an output swing range of 0V to V_CC – 1.2V.
The output uses an NPN emitter follower without any
internal pull-down current. A DC resistive load to ground
is required in order to sink current.
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, allowing remote voltage sensing.
3731fa
17