LT3587 데이터 시트보기 (PDF) - Linear Technology

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LT3587

High Voltage Monolithic Inverter and Dual Boost

Linear Technology 

LT3587

APPLICATIONS INFORMATION

FLT B1 B3 SW3 VIN SW1

CAP3 EN/SS EN/SS

GND

RVFB3

VFB3

VOUT3

LT3587

IFB3

SW2 DN FB2

CAP1

FB1

VOUT1

RFB1

RVFB3

FLT B1 B3 SW3 VIN SW1

CAP3 EN/SS EN/SS

GND

CAP1

VFB3

LT3587

VOUT3

FB1

IFB3

SW2 DN FB2

VOUT1

RFB1

3587 F09

Figure 9. Feedback Connection Using the VOUT and CAP Pins

However, in the case of a short-circuit fault at the VOUT

pins, the LT3587 will switch continuously because the FB1

or the VFB3 pin is low. While operating in this open-loop

condition, the rising voltage at the CAP pins is limited

only by the protection circuit of their respective output

disconnects. At the worst case, the CAP pin rises to 10V

above the corresponding VOUT pin. So in the case of short-

circuit fault to ground, the voltage on the CAP pins may

reach 10V. When the short-circuit condition is removed,

the VOUT pins rise up to the voltage on the CAP pins,

potentially exceeding the programmed output voltage until

the capacitor voltages fall back into regulation. While this

is harmless to the LT3587, this should be considered in

the context of the external circuitry if short-circuit events

are expected.

Regulating the CAP pins ensures that the voltage on the

VOUT pins never exceeds the set output voltage after a

short-circuit event. However, this setup does not com-

pensate for the voltage drop across the output disconnect,

resulting in an output voltage that is slightly lower than

the voltage set by the feedback resistor. This voltage drop

is equal to the product of the output current and the on

resistance of the PMOS disconnect transistor. This drop

can be accounted for when using the CAP pin as the

feedback node by setting the output voltage according to

the following formula:

RFB1 =

VVOUT1

+ IVOUT1 • RDISC1

13.8μA

− 1.22V

RFB3

=

VVOUT3

+ IVOUT3 • RDISC3

14.3μA

−

0.8V

Fault Detection and Indicator

The LT3587 features fault detection on all its outputs and

a fault indicator pin (FLT). The fault detection circuitry is

enabled only when at least one of the channels has com-

pleted the soft-start process and is free running with full

inductor current. Once the fault detection is enabled, the

Fault pin pulls low when any of the feedback voltages (VFB1,

VFB2 or Max(VVFB3,VIFB3)) fall below their regulation value

for more than 16ms.

One particularly important case is an overload or short-

circuit condition on any of the channel outputs. In this case,

if the corresponding loop is unable to bring the output

back into regulation within 16ms, a fault is detected and

the Fault pin is pulled low.

Note that the fault condition is latched. Once the Fault pin

is pulled low, all the three channels are disabled. In order

to enable any of the channels again, reset the part by shut-

ting it down and then turning it on again. This is done by

first forcing both the EN/SS1 and EN/SS3 pins low below

200mV and then either letting them go high again in a

soft-start process or forcing them high immediately if no

soft-start is desired. Figure 10 shows the waveforms when

a short-circuit condition occurs at Boost1 for more than

16ms as well as the subsequent resetting of the part.

VFLT

5V/DIV

ENSS1/ENSS3

5V/DIV

VVOUT1

10V/DIV

VNEG

10V/DIV

VVOUT3

20V/DIV

SHORT

AT VOUT1

PART RESET

100ms/DIV

3587 F10

Figure 10. Waveforms During Fault

Detection of a Short-Circuit Event

3587fc

15

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