LTC3738 Linear Technology, LTC3738 Datasheet - Page 19

• Manufacturer Part Number: LTC3738
• Description: 3-Phase Buck Controller
• Manufacturer: Linear Technology

w w w . D a t a S h e e t 4 U . c o m
APPLICATIO S I FOR ATIO
V
The V
of the controller but also to the top and bottom gate
drivers and therefore must be bypassed very carefully to
ground with a ceramic capacitor, type X7R or X5R (de-
pending upon the operating temperature environment) of
at least 1 µ F imme diately 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
External bootstrap capacitors, C
pins, supply the gate drive voltages for the topside
MOSFETs. Capacitor C
charged though diode D
low. When one of the topside MOSFETs turns on, the
driver places the C
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
on, the boost voltage is above the input supply (V
V
30 to 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of D
greater than V
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
CC
IN
CC
and the BOOST pin follows. With the topside MOSFET
+ V
Decoupling
CC
IN
pin supplies power not only to the internal circuits
). The value of the boost capacitor C
IN(MAX).
B
U
voltage across the gate-source of the
B
B
U
in the Functional Diagram is
from V
B
, connected to the BOOST
CC
W
B
, D
when the SW pin is
B
)
B
needs to be
U
B
must be
BOOST
=
common, tightly coupled pair of PC traces. The differen-
tial amplifier rejects common mode signals capacitively
or inductively radiated into the feedback PC traces as well
as ground loop disturbances. The differential amplifier
output signal is divided down through the VID DAC and
is compared with the internal, precision 0.6V voltage
reference by the error amplifier.
The amplifier has a 0 to V
and an output swing range of 0 to V
uses an NPN emitter follower with 160kΩ feedback
resistance.
Output Voltage
Selection of the VRM9 or VRM10 VID table is through the
VID5 pin. Tying VID5 to V
If the VRM9 VID table is selected (Table 1), output voltage
in 25mV increments is produced from 1.1V to 1.85V.
There is a built-in –12.5mV DC offset for the output
voltage.
If the VRM10 VID table is selected (Table 2), output voltage
in 12.5mV increments is produced from 0.8375V to 1.6V.
There is a built-in –25mV DC offset for output voltage.
Active Voltage Position Control
The LTC3738 senses inductor current information through
monitoring voltage drops on the sense resistor R
all three channels. The voltage drops are added together
and applied as V
which are connected through resistor R
V
voltage as the compensation for the load voltage drop. In
summary, the load slope is:
The recommended value for R
PRE-AVP
⎛
⎜
⎝
R
SENSE
is scaled through R
•
R
PRE AVP
R
PRE-AVP
AVP
−
⎞
⎟
⎠
CC
between the AVP and IN
CC
V A
will select the VRM9 VID table.
/
common mode input range
AVP
AVP
is 90Ω to 100Ω.
and added to output
CC
– 1.2V. The output
LTC3738
PRE-AVP
SENSE
19
+
. Then
pins,
3738f
of