19
LTC3738
3738f
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 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
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
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
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 160kΩ feedback
resistance.
Output Voltage
Selection of the VRM9 or VRM10 VID table is through the
VID5 pin. Tying VID5 to V
CC
will select the VRM9 VID table.
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
SENSE
of
all three channels. The voltage drops are added together
and applied as V
PRE-AVP
between the AVP and IN
+
pins,
which are connected through resistor R
PRE-AVP
. Then
V
PRE-AVP
is scaled through R
AVP
and added to output
voltage as the compensation for the load voltage drop. In
summary, the load slope is:
R
R
R
VA
SENSE
AVP
PRE AVP
•/
−
⎛
⎝
⎜
⎞
⎠
⎟
The recommended value for R
AVP
is 90Ω to 100Ω.
APPLICATIO S I FOR ATIO
WUUU
