LTC1702IGN datasheet(16/36 Pages) LINER

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LTC1702

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.

Note that in overvoltage conditions, the MAX comparator

will kick in at just +5%, turning QB on continuously long

before the output reaches +15%. Under most fault condi-

tions, this is adequate to bring the output back down

without firing the fault latch. Additionally, if MAX success-

fully keeps the output below +15%, the LTC1702 will

resume normal regulation as soon as the output overvolt-

age fault is resolved.

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 LTC1702

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 10

µA current source pull-up will

not be able to pull FAULT high, and the LTC1702 will ignore

the latch and continue normal operation. The MAX com-

parator will act as usual, turning on QB until output is

within range and then allowing the loop to resume normal

operation. FAULT can also be pulled down with external

open-collector logic to restart a fault-latched LTC1702 as

an alternative to recycling the power. Note that this will not

reset the internal latch; if the external pull-down is

released, the LTC1702 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 LTC1702 depends strongly

on the external MOSFETs used. The LTC1702 requires at

least two external MOSFETs per side—more if one or

more of the MOSFETs are paralleled to lower on-resis-

tance. To work efficiently, these MOSFETs must exhibit

low RDS(ON) at 5V VGS (3.3V VGS if the PVCC input supply

APPLICATIONS INFORMATION

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 the

other hand, voltage breakdown requirements in a typical

LTC1702 circuit are pretty tame: the 7V maximum input

voltage limits the VDS and VGS the MOSFETs can see to

safe levels for most devices.

Low RDS(ON)

RDS(ON) calculations are pretty straightforward. RDS(ON) is

the resistance from the drain to the source of the MOSFET

when the gate is fully on. Many MOSFETs have RDS(ON)

specified at 4.5V gate drive—this is the right number to

use in LTC1702 circuits running from a 5V supply. As

current flows through this resistance while the MOSFET is

on, it generates I2R watts of heat, where I is the current

flowing (usually equal to the output current) and R is the

MOSFET RDS(ON). This heat is only generated when the

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

RDS(ON) improves heavy load efficiency at the expense of

additional gate charge (usually) and more cost (usually).

Proper choice of MOSFET RDS(ON) becomes a trade-off

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 LTC1702 circuit, allowing the use of small, surface

mount MOSFETs without heat sinks.

Gate Charge

Gate charge is amount of charge (essentially, the number

of electrons) that the LTC1702 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

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