LTC4366 datasheet(12/22 Pages) AD

12 / 22 page

LTC4249

Rev. 0

For more information www.analog.com

Applying the RDY Output

The RDY open-drain output is released and allowed to pull

high when the ECB is fully turned on and OUT is “ready”

for application. In a RF PA circuit, with OUT connected to

the PA drain, RDY may be used to enable PA gate bias.

RDY pulls low when EN is low and/or when fault events

such as overcurrent, short-circuit or thermal shutdown

conditions have occurred.

Figure 7 shows a typical RDY response to EN and subse-

quent overcurrent fault. RDY may be pulled up as high as

5.5V, by an external source. Resistors in the range of 1k to

10k are recommended. RDY may also be used to enable

other downstream loads such as DC-DC converters.

OVERCURRENT EVENT

10ms/DIV

5V/DIV

OUT

20V/DIV

RDY

5V/DIV

IMON

2A/DIV

4249 F07

Figure 7. Typical RDY Response

Configuring Overcurrent (OC) Thresholds

Overcurrent thresholds are easily configured with a sin-

gle resistor (RIMON) from IMON to GND. IMON sources

100µA/A of ECB current. The primary (filtered) OC voltage

threshold at the IMON output is 1.2V. RIMON is simply

determined from:

RIMON =

1.2V

IECB

•10,000

where IECB is the desired circuit breaker threshold in

amperes. For a 1.2A circuit breaker threshold, RIMON is

10kΩ. Larger values of RIMON yield lower circuit breaker

thresholds. The secondary (fast) OC threshold is always

twice the configured primary OC threshold.

The primary OC threshold is filtered to remove small

fluctuations which allows the ECB to operate at currents

near the OC threshold. In order to trip the primary OC

comparator, the 1.2V threshold must be overdriven with

requisite magnitude and duration. The typical overdrive

versus time characteristic for the primary OC comparator

is shown in Figure 8.

ECB TURNS OFF ABOVE CURVE

ECB REMAINS ON BELOW CURVE

PRIMARY COMPARATOR OVERDRIVE (V)

0.01

0.1

100

500

Primary Comparator

Response vs Overdrive

4249 F08

Figure 8. Primary OC Comparator Filter Characteristic

The secondary OC threshold comparator does not have

added filtering and has a 2µs (typical) response time when

the 2.4V IMON threshold has been exceeded.

External filtering of the IMON output voltage to achieve

added ECB delay or noise rejection can be performed by

adding a capacitor CIMON in parallel with RIMON. A pole

will be formed by the RC combination.

Monitoring ECB Current

The IMON output current provides an accurate scaled

replica (100µA/A) of the OUT current. The IMON output

current is 5% accurate (30mA to 1.2A) over temperature

(Figure 9). Note that excellent accuracy is maintained even

at ECB currents below 30mA. The IMON accuracy reduces

the need to over design the input power supply current

capability due to the low variability in ECB threshold.

APPLICATIONS INFORMATION

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