LTC4367 datasheet(10/22 Pages) AD

10 / 22 page

LTC4249

Rev. 0

For more information www.analog.com

With an internal 0.8V EN threshold, and a desired 10.8V

UV threshold, R1 may be easily determined by specifying

the desired resistive divider load current. Picking 100µA

as a starting point, the relationship for R1 is:

R1=

10.8V −0.8V

100µA

=100kΩ

R2 is determined by dividing the 0.8V EN threshold by the

divider load current:

0.8V

100µA

= 8kΩ

The closest standard 1% resistor is 8.06k. The resistor

difference will not cause much error in the UV threshold

(10.73V).

To prevent ECB chattering due to a noisy monitored volt-

age, the 0.8V EN threshold has 35mV falling hysteresis.

The supply referred hysteresis is simply related to the

resistive divider gain:

Falling Hysteresis

= 35mV• 1+

⎛

⎝⎜

⎞

⎠⎟

= 469mV

Therefore, for the application in Figure 2, after the moni-

tored supply rises above 10.73V, the falling UV threshold

drops to 10.26V. If the monitored supply is below the

configured UV threshold the ECB channel is turned off.

Automatic Inrush Control

The LTC4249 is designed to control the inrush current

upon enabling an ECB channel. For the first 10ms, out-

put loads are charged at 1A. Maximum capacitive load is

100µF. At 100µF, the output charges at 10V/ms. When

operating at the maximum input supply of 65V, a 100µF

capacitor charges in 6.5ms, and is safely below the 10ms

charge time. Figure 3 demonstrates output charging at

maximum operating voltage and capacitive load.

OUT Charging 100µF (IN = 65V)

tCL

5ms/DIV

5V/DIV

OUT

50V/DIV

RDY

5V/DIV

4249 F03

Figure 3. Output Charging at Initial Turn-On

Attempting to charge a large capacitance quickly requires

large currents. Without inrush control, the safe operating

area of the ECB would be exceeded. Furthermore, without

inrush control, voltage sag on the input supply could be

severe, causing brown-outs or system resets.

All LTC4249 applications require that the ECB output be

fully charged (VOUT ≈ VIN) within 10ms. After the 10ms

current limited operating phase (tCL), the ECB turns on

fully to its rated 75mΩ on-resistance. At this point, over-

current protection becomes active and the RDY output

is released.

Voltage or Capacitance Start-Up Derating at High

Ambient Temperature

The LTC4249 is designed to prevent operation outside

its Safe Operating Area (SOA). Maximum power dissipa-

tion occurs when starting both ECB channels simulta-

neously. During the inrush portion of operation, with an

ambient temperature above 90°C, junction temperature

may briefly exceed the thermal shutdown threshold above

150°C, depending on input voltage and output capaci-

tance. Exceeding the thermal shutdown threshold causes

the ECBs to safely latch off. Toggling the enable inputs

allows the ECBs to turn back on.

Figure 4 shows the allowable operating time at a given

power dissipation before reaching thermal shutdown.

Operating to the left of the curves prevents thermal shut-

down. After inrush, typical power dissipation drops below

0.5W, which keeps the junction temperature far below

thermal shutdown.

APPLICATIONS INFORMATION

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