LTM4656 datasheet(12/28 Pages) AD

12 / 28 page

LTM4656/LTM4656-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 4 and Figure 5 show the overcurrent fault wave-

forms, and the timer pin timeout period as a function of the

amount of voltage across the MIN power MOSFET during

an overcurrent fault. Figure 4 shows a 10V to 36V boost

in short-circuit, and Figure 5 shows a 28V to 36V boost in

a short-circuit. Figure 2 shows that the over current timer

(TIMER pin) will expire at ~240µs in a short-circuit at the

1.375V threshold with 10V input, will expire at 73µs at 40V

input. Figure 4 timeout with 10V input and Figure 5 time out

with 28V input are very close to Figure 2 time out period

relative to input voltage. These timeout periods across the

full input range can be checked against the Figure 3 MIN

MOSFET SOA curves to ensure adequate margin relative

to voltage across the input protection MOSFET and the

current flowing through. Adequate guard band is to ensure

that these conditions are OK overtemperature.

Figure 4. 10V Input to 36V Output Short Input Current

Trip Waveform

TURN-ON INRUSH CURRENT CONTROL AND

POWER-UP INTO LOAD

TheLTM4656turnsontheMINpowerMOSFETwithatime

control ramp of 10ms. When the voltage at the SENSE1

pin is within 0.5V of VIN and 3V above GND, indicating

the external MOSFET is fully on, then the internal ENABLE

signal goes high impedance to allow the RUN pin to acti-

vate the boost converter. The state of the internal ENABLE

signal is latched until the SENSE1 pin voltage drops below

2V, resetting the latch. Utilizing the UV pin to set a UVLO

voltage before turn on, then having the boost converter

turn on after the MIN MOSFET is fully enhanced, and the a

0.1µF soft-start capacitor will assure no false overcurrent

trips at start-up.

SYNCHRONOUS BOOST CONVERTER SECTION

The LTM4656 has a high power synchronous converter

downstream of the input protection. A synchronous boost

converterinherentlyhasdifficultywithoutputshort-circuit

due to the MOUT output power MOSFET body diode

conducting.

The input protection path will control the input current to

boost converter and auto-retry during an output short.

The LTM4656 uses a constant-frequency, current mode

step-up control architecture. During normal operation, the

MBOTbottomMOSFETisturnedonwhentheclocksetsthe

internal RS latch, and is turned off when the main (ICMP)

current comparator resets the RS latch. The peak induc-

tor current at which ICMP trips and resets the latch is

controlled by the voltage on the COMP pin, which is the

output of the error amplifier. The error amplifier compares

the output voltage feedback signal at the VFB pin.

The LTM4656-1 provides optional External Compensation.

In a boost converter, the required inductor current is de-

termined by the load current, VIN and VOUT. When the load

currentincreases,itcausesaslightdecreaseinVFBrelative

to the reference, which causes the error amp to increase

the COMP voltage until the average inductor current in

the channel matches the new requirement based on the

new load current. After the bottom MOSFET is turned off

Figure 5. 28V Input to 36V Output Short Input Current

Trip Waveform

200µs/DIV

4656 F04

*25mV SENSE IN FOLDBACK

TIMER (78 s)

VIN

5V/DIV

VOUT

5V/DIV

IIN

5A/DIV

TIMER PIN

1V/DIV

VOUT SHORTED

~240 SEC TIME OUT

INPUT CURRENT FOLDBACK TO 5A

100µs/DIV

VIN

20V/DIV

VOUT

20V/DIV

4656 F05

*25mV SENSE IN FOLDBACK

TIMER (218 s)

IIN

5A/DIV

TIMER PIN

1V/DIV

VOUT SHORTED

~100 SEC TIME OUT

INPUT CURRENT FOLDBACK TO 5A

LTM4656 数据表(PDF) 12 Page - Analog Devices

LTM4656 数据表(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表下载