LTC3733_15 Datasheet, PDF (18/32 Pages) Linear Technology – 3-Phase, Buck Controllers for AMD CPUs

LTC3733/LTC3733-1
APPLICATIO S I FOR ATIO
series of surface-mount tantalums or the Panasonic SP
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 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:
R_SENSE
=
N
50mV
I_MAX
The IC works well with values of R_SENSE from 0.001Ω to
0.02Ω.
V_CC Decoupling
The V_CC pin supples power not only the internal circuits of
the controller but also the top and bottom gate drivers and
therefore must be bypassed very carefully to ground with
a ceramic capacitor, type X7R or X5R (depending 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 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 input capacitance of the
topside MOSFET(s). The reverse breakdown of D_B must be
greater than V_IN(MAX).
Differential Amplifier
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 through the VID DAC and is
compared with the internal, precision 0.6V voltage refer-
ence by the error amplifier.
The amplifier has a 0 to V_CC common mode input range
and an output swing range of 0 to V_CC – 1.2V. The output
uses an NPN emitter follower with 80kΩ feedback resis-
tance. A DC resistive load to ground is required in order to
sink more current.
3733f
18