LTC4280CUFD datasheet(17/28 Pages) LINER

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LTC4280

4280f

APPLICATIONS INFORMATION

pin or bit A3 going from high to low, if the UV pin is brought

below its 0.4V reset threshold for 2μs, or if INTVCC falls

below its 2.64V undervoltage lockout threshold. Finally,

when EN is brought from high to low, only FAULT bits

D0-D3 are cleared, and bit D4, that indicates a EN change

of state, is set. Note that faults that are still present, as

indicated in STATUS Register C, cannot be cleared.

The FAULT register is not cleared when auto-retrying.

When auto-retry is disabled the existence of a D0, D1

or D2 fault keeps the switch off. As soon as the fault is

cleared, the switch turns on. If auto-retry is enabled, then

a high value in C0, C1 or C2 holds the switch off and the

fault register is ignored. Subsequently, when bits C0, C1

and C2 are cleared by removal of the fault condition, the

switch is allowed to turn on again.

The LTC4280 will set bit D2 and turn off in the event of

an overcurrent fault, preventing it from remaining in an

overcurrent condition. If conﬁgured to auto-retry, the

LTC4280 will continually attempt to restart after cool-down

cycles until it succeeds in starting up without generating

an overcurrent fault.

Data Converter

The LTC4280 incorporates an 8-bit Δ∑ A/D converter

that continuously monitors three different voltages. The

Δ∑ architecture inherently averages signal noise during

the measurement period. The SOURCE pin has a 1/12.5

resistive divider to monitor a full-scale voltage of 15.4V

with 60mV resolution. The ADIN pin is monitored with a

1.235V full-scale and 4.82mV resolution, and the voltage

between the VDD and SENSE pins is monitored with a

38.6mV full-scale and 151μV resolution.

Results from each conversion are stored in registers E

(Sense), F (Source) and G (ADIN), as seen in Tables 6-8,

and are updated 10 times per second. Setting CONTROL

converter so that registers E, F, and G may be written to

and read from for software testing.

Conﬁguring the GPIO Pin

Table 2 describes the possible states of the GPIO pin using

the control register bits A6 and A7. At power-up, the default

state is for the GPIO pin to go high impedance when power

is good (FB pin greater than 1.235V). Other applications

for the GPIO pin are to pull down when power is good, a

general purpose output and a general purpose input.

Current Limit Stability

For many applications the LTC4280 current limit will be

stable without additional components. However there

are certain conditions where additional components

may be needed to improve stability. The dominant pole

of the current limit circuit is set by the capacitance and

resistance at the gate of the external MOSFET, and larger

gate capacitance makes the current limit loop more stable.

Usually a total of 8nF gate to source capacitance is sufﬁcient

for stability and is typically provided by inherent MOSFET

CGS, however the stability of the loop is degraded by

increasing RSENSE or by reducing the size of the resistor

on a gate RC network if one is used, which may require

additional gate to source capacitance. Board level short-

circuit testing in highly recommended as board layout can

also affect transient performance, for stability testing the

worst case condition for current limit stability occurs when

the output is shorted to ground after a normal startup.

There are two possible parasitic oscillations when the

MOSFET operates as a source follower when ramping

at power-up or during current limiting. The ﬁrst type of

oscillation occurs at high frequencies, typically above

1MHz. This high frequency oscillation is easily damped

with R5 as shown in Figure 1. In some applications, one

may ﬁnd that R5 helps in short-circuit transient recovery

as well. However, too large of an R5 value will slow down

the turn-off time. The recommended R5 range is between

5Ω and 500Ω.

The second type of source follower oscillation occurs at

frequencies between 200kHz and 800kHz due to the load

capacitance being between 0.2μF and 9μF, the presence

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