LTC1702ACGN#TR-SMI Linear Technology, LTC1702ACGN#TR-SMI Datasheet - Page 16

• Manufacturer Part Number: LTC1702ACGN#TR-SMI
• Manufacturer: Linear Technology

Specifications of LTC1702ACGN#TR-SMI

• Lead Free Status / RoHS Status: Not Compliant

LTC1702A
APPLICATIONS
remains in this state until both RUN/SS pins are pulled low
simultaneously, the power supply is recycled, or the
FAULT pin is pulled low externally. This behavior is in-
tended to protect a potentially expensive load from over-
voltage damage at all costs. Under some conditions, this
behavior can cause the output voltage to undershoot
below ground. If latched FAULT mode is used, a Schottky
diode should be added with its cathode at the output and
its anode at ground to clamp the negative voltage to a safe
level and prevent possible damage to the load and the
output capacitors.
In some circuits, the OV latch can be a liability. Consider
a circuit where the output voltage at one channel may be
changed on the fly by switching in different feedback
resistors. A downward adjustment of greater than 15%
will fire the fault latch, disabling both sides of the LTC1702A
until the power is recycled. In circuits such as this, the fault
latch can be disabled by grounding the FAULT pin. The
internal latch will still be set the first time the output
exceeds +15%, but the 10µA current source pull-up will
not be able to pull FAULT high, and the LTC1702A will
ignore the latch and continue normal operation. FAULT
can also be pulled down with external open-collector logic
to restart a fault-latched LTC1702A as an alternative to
recycling the power. Note that this will not reset the
internal latch; if the external pull-down is released, the
LTC1702A will reenter FAULT mode. To reset the latch,
pull both RUN/SS pins low simultaneously or cycle the
input power.
EXTERNAL COMPONENT SELECTION
POWER MOSFETs
Getting peak efficiency out of the LTC1702A depends
strongly on the external MOSFETs used. The LTC1702A
requires at least two external MOSFETs per side—more if
one or more of the MOSFETs are paralleled to lower on-
resistance. To work efficiently, these MOSFETs must
exhibit low R
supply is 3.3V) to minimize resistive power loss while they
are conducting current. They must also have low gate
charge to minimize transition losses during switching. On
16
DS(ON)
U
at 5V V
INFORMATION
U
GS
(3.3V V
W
GS
if the PV
U
CC
input
the other hand, voltage breakdown requirements in a
typical LTC1702A circuit are pretty tame: the 7V maximum
input voltage limits the V
to safe levels for most devices.
Low R
R
the resistance from the drain to the source of the MOSFET
when the gate is fully on. Many MOSFETs have R
specified at 4.5V gate drive—this is the right number to
use in LTC1702A circuits running from a 5V supply. As
current flows through this resistance while the MOSFET is
on, it generates I
flowing (usually equal to the output current) and R is the
MOSFET R
MOSFET is on. When it is off, the current is zero and the
power lost is also zero (and the other MOSFET is busy
losing power).
This lost power does two things: it subtracts from the
power available at the output, costing efficiency, and it
makes the MOSFET hotter—both bad things. The effect is
worst at maximum load when the current in the MOSFETs
and thus the power lost are at a maximum. Lowering
R
additional gate charge (usually) and more cost (usually).
Proper choice of MOSFET R
between tolerable efficiency loss, power dissipation and
cost. Note that while the lost power has a significant effect
on system efficiency, it only adds up to a watt or two in a
typical LTC1702A circuit, allowing the use of small, sur-
face mount MOSFETs without heat sinks.
Gate Charge
Gate charge is amount of charge (essentially, the number
of electrons) that the LTC1702A needs to put into the gate
of an external MOSFET to turn it on. The easiest way to
visualize gate charge is to think of it as a capacitance from
the gate pin of the MOSFET to SW (for QT) or to PGND (for
QB). This capacitance is composed of MOSFET channel
charge, actual parasitic drain-source capacitance and Miller-
multiplied gate-drain capacitance, but can be approximated
as a single capacitance from gate to source. Regardless of
DS(ON)
DS(ON)
DS(ON)
calculations are pretty straightforward. R
improves heavy load efficiency at the expense of
DS(ON)
2
. This heat is only generated when the
R watts of heat, where I is the current
DS
and V
DS(ON)
GS
the MOSFETs can see
becomes a trade-off
DS(ON)
DS(ON)
1702afa
is