LTC3731HGTR Linear Technology, LTC3731HGTR Datasheet - Page 17

• Manufacturer Part Number: LTC3731HGTR
• Manufacturer: Linear Technology

Specifications of LTC3731HGTR

• Lead Free Status / RoHS Status: Not Compliant

APPLICATIO S I FOR ATIO
available 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
Once the frequency and inductor have been chosen,
R
required peak inductor current. The current comparator
has a typical maximum threshold of 75mV/R
input common mode range of SGND to (1.1) • V
current comparator threshold sets the peak inductor cur-
rent, yielding a maximum average output current I
equal to the peak value less half the peak-to-peak ripple
current, ∆I
Allowing a margin for variations in the IC and external
component values yields:
The IC works well with values of R
0.02Ω.
V
The V
of the controller and to the top and bottom gate drivers.
Therefore, they must be bypassed very carefully to ground
with ceramic capacitors, type X7R or X5R (depending
upon the operating temperature environment) of at least
1 µ F imme diately next to the IC and preferably an addi-
tional 10µF placed very close to the IC due to the extremely
high instantaneous currents involved. The total capaci-
tance, taking into account the voltage coefficient of ce-
ramic 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.
CC
SENSE
SENSE1,
R
Decoupling
SENSE
CC
Selection for Output Current
and V
R
L
SENSE2,
=
.
N
DR
50
I
MAX
pins supply power to the internal circuits
mV
R
U
SENSE3
U
are determined based on the
SENSE
W
from 0.002Ω to
SENSE
U
CC
and an
. The
MAX
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 gate charge capacitance of
the topside MOSFET(s) as specified on the manufacturer’s
data sheet. The reverse breakdown of D
than V
The output voltage is set by an external resistive divider
according to the following formula:
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
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.
IN
CC
V
and the BOOST pin follows. With the topside MOSFET
+ V
OUT
IN(MAX).
IN
=
). The value of the boost capacitor C
0 6 1
.
V
⎛
⎜
⎝
B
+
voltage across the gate-source of the
R
R
2
1
⎞
⎟
⎠
B
B
in the Functional Diagram is
from V
B
, connected to the BOOST
CC
B
, D
when the SW pin is
LTC3731H
B
B
)
must be greater
B
needs to be
ITH
BOOST
17
). The
3731hfa
=