LTC4367 datasheet(11/22 Pages) AD

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LTC4249

Rev. 0

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BOTH CHANNELS

STARTING TOGETHER

TA = 125°C

TA = 100°C

TA = 25°C

TA = –40°C

POWER DISSIPATION (W)

0.1

100

0.001

0.01

0.1

Thermal Shutdown Time

vs Power Dissipation

4249 F04

Figure 4. Thermal Shutdown Time vs Power Dissipation

Figure 5 provides temperature derating information for

input voltage and output capacitance during the inrush

portion of operation. If the output capacitance is 47µF (or

less), 65V operation is possible at any allowed ambient

temperature. At 100µF, input voltage should be reduced

by –375mV/°C, starting at 90°C ambient. Connecting the

exposed back pad to a ground plane is recommended for

heat removal.

BOTH CHANNELS

STARTING TOGETHER

47µF

100µF

TA (°C)

80 85 90 95 100 105 110 115 120 125

Start-Up Voltage Derating vs

Ambient Temperature and Capacitor Load

4249 F05

Figure 5. High Temperature Start-Up Derating

Short-Circuit Protection

During the first 300µs of the 10ms current limited operat-

ing phase, the OUT voltage is monitored for voltage rise.

If the OUT voltage does not exceed 1.2V at the 300µs time

point, OUT is considered shorted and the ECB turns off.

At the conclusion of the 10ms current limited operating

phase, the short-circuit logic is inverted. If OUT should

fall below 1.2V, OUT is again considered shorted and the

ECB turns off. The short-circuit comparator response time

is typically 2µs for 50mV overdrive.

An important applications consideration under a short-

circuit event is the amount of inductance between the ECB

outputs and the load. An antenna array with multiple RF

PAs could have inches of trace inductance between the

ECB channel outputs and each PA. It is recommended to

keep load path inductance below 400nH.

When the ECB is on and the load end is shorted through

an inductance as shown in Figure 6, the ECB current

increases rapidly. Peak current is a function of the out-

put voltage, trace inductance and the time to turn off the

ECB. The secondary OC and short-circuit comparators

have limited bandwidth (about 2µs response time) and

are not sufficiently fast enough to limit the current build

up to typical OC setpoints under an inductive hard short.

However, the ECB controller itself does have limited cur-

rent compliance, allowing the OUT voltage to fall, keep-

ing the ECB inside its safe operating area. After reaching

peak current, the ECB dissipates the inductor energy and

turns off. The integrated protections eliminate the need

for large high-power and high-voltage Schottky diodes

at the OUT pin.

LTC4249

VDRAIN

TRACE INDUCTANCE

(≤12")

OUT

SHORT-

CIRCUIT

+–

4249 F06

Figure 6. Load Short with Inductance

The input voltage side of the ECB should also have low

inductance. For array applications, a large low impedance

layout is recommended for input voltage distribution. It is

recommended to keep the input supply path inductance

below 400nH. A 1µF (or greater) bypass capacitor placed

close to the IN inputs and ground is recommended.

APPLICATIONS INFORMATION

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