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LT3587EUD

High Voltage Monolithic Inverter and Dual Boost

Linear Technology 

LT3587

APPLICATIONS INFORMATION

current at VOUT3 to ILIMIT. Figure 4 compares the transient

responses with and without current limit when a current

overload occurs.

VVOUT3 15V

5V/DIV

IVOUT3

13mA/DIV

20mA

IL4

200mA/DIV

LOAD STEP

VVIN = 3.6V

200μs/DIV

WITHOUT CURRENT LIMIT: IFB3

CONNECTED TO GND

3587 F04a

VOUT3 STAYS AT 15V, OUTPUT CURRENT

INCREASES FROM 20mA TO 40mA

lower than 29V is obtained by connecting a resistor from

the VOUT3 pin to the VFB3 pin such that:

RFB3 = ((VCLAMP/0.8V) – 1) • 56.3k

where VCLAMP is the desired output voltage clamp level. In

this case, when the voltage level is less than VCLAMP, the

Boost3 loop regulates the voltage at the IFB3 pin to 0.8V.

When the output load fails open-circuit or is disconnected,

the voltage at IFB3 drops to reflect the lower output current

and the voltage at VFB3 starts to rise. When the voltage

at VOUT3 rises to VCLAMP, the Boost3 loop then regulates

the voltage at the VFB3 pin to 0.8V, limiting the voltage

level at VOUT3 to VCLAMP. Figure 5 contrasts the transient

responses with and without programmed VCLAMP when

the output load is disconnected.

VVOUT3

15V

5V/DIV

IVOUT3

13mA/DIV

20mA

IL4

200mA/DIV

LOAD STEP

VVIN = 3.6V

200μs/DIV

3587 F04b

WITH 20mA CURRENT LIMIT: RIFB3 = 8.06k

OUTPUT CURRENT STAYS AT 20mA,

VOUT3 DROPS FROM 15V TO 7.5V

Figure 4. Boost3 Waveform in an Output Current

Overload Event with and Without Output Current Limit

The LT3587 CAP3 pin has an internal overvoltage protec-

tion. When the voltage at the CAP3 pin is driven above

29V (typ), the Boost3 loop is disabled and the SW3 pin

stops switching.

When configured as a boost current regulator, a feedback

resistor from the IFB3 pin to ground sets the output cur-

rent at VOUT3 at a fixed level. In this case, if the VFB3 pin is

grounded then the overvoltage protection defaults to the

open-circuit clamp voltage level of 29V. A voltage clamp

VVOUT3 20V

10V/DIV

IL4

200mA/DIV

OUTPUT LOAD

DISCONNECTED

VVIN = 3.6V

200μs/DIV

3587 F05a

WITHOUT PROGRAMMED OUTPUT VOLTAGE

CLAMP: VFB3 CONNECTED TO GND

VVOUT3 20V

10V/DIV

IL4

200mA/DIV

OUTPUT LOAD

DISCONNECTED

VVIN = 3.6V

200μs/DIV

WITH PROGRAMMED OUTPUT

VOLTAGE CLAMP AT 24V

3587 F05b

Figure 5. Boost3 Output Open-Circuit Waveform with

and Without Programmed Output Voltage Clamp

3587fc

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