DC694 - LT4351MS10 Evaluation Kit Quick Start Guide

DC694 - LT4351MS10 Evaluation Kit Quick Start Guide
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 694
ORED POWER SUPPLY DEMO CIRCUIT
LT4351
DESCRIPTION
Demonstration circuit DC694 is an ORed Power Supply Demo Circuit featuring the LT®4351. The demo
circuit is intended to allow for 3 diode ORed supplies.
Two supplies are ORed with the LT4351 MOSFET Diode-OR Controller while the third uses a Schottky diode for comparison.
The LT4351 circuits on this assembly are set up for
use with 12V supplies.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
Table 1. Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Maximum Input Voltage
19V
Maximum Output Current
Limited by pass elements
Typically 7.5A
Nominal VDD1 or VDD2 voltage
Generated by on chip boost regulator
(V1 or V2)+10.7V
Overvoltage trip voltage for LT4351 channels
15.1V
Undervoltage trip voltage for LT4351 channels
9.6V
Turn on voltage for LT4351 channels
10.2V
Undervoltage hysteresis for LT4351 channels
0.6V
OPERATING PRINCIPLES
DC694 uses the LT4351 controller (U1 and U2) to
create a nearly ideal diode function for two channels.
The third channel is a Schottky diode (D5).
The LT4351 creates the ideal diode function by turning on low RDSON N-channel MOSFETs when the input supply is greater than the output. When the input
source voltage drops below the output common supply voltage it turns off the MOSFET, thereby matching
the function and performance of an ideal diode.
The demo circuit uses dual N-channel MOSFETs in a
single package, wired back-to-back (Q3 and Q6).
Back-to-back MOSFETs are used to prevent current
flow in the MOSFET body diode during an overvoltage
condition.
The LT4351 gate driver amplifier monitors the input
(V1 or V2) and output (OUT) and controls the MOSFETs. If the input voltage exceeds OUT by 15mV, the
LT4351 turns on the MOSFETs allowing current to
flow from VIN to OUT.
Additionally the LT4351 can inhibit current flow when
the input supply is out of range either high or low.
This is done with on chip undervoltage and overvoltage comparators. Resistive dividers (R5/R6, RA1/RB1
and R11/R12, RA2/RB2) in conjunction with the
LT4351 UV and OV pins sets appropriate thresholds
such that the MOSFETs are off when the UV pin is
below 300mV or OV pin is above 300mV.
To help deal with the transients on the supply lines,
the UV input has current hysteresis. When the UV
voltage drops below the 300mV threshold, a 10µA
current is pulled from the pin. Thus the user can set
the hysteresis level through appropriate values in the
divider. For the demo circuit the resistive dividers set
a 9.6V undervoltage and 15.1V overvoltage.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 694
ORED POWER SUPPLY DEMO CIRCUIT
LED’s are provided to indicate the states of the controller. The green STATUS LEDs (D1, D6) will light
when the MOSFETs are most likely on and passing
current ((V1 or V2) > OUT + 15mV and GATE > (V1 or
V2) + 0.7V).
The red FAULT LEDs (D2, D7) turn on when the respective channel is out of range because of overvoltage or undervoltage condition.
The FAULT LED can also light when there may be a
problem with the MOSFETs such that maximum gate
drive is being provided and the drop across the MOSFETs is greater than 0.21V. This can occur if the MOSFETs fail or if the I* RDSON is greater than 0.21V. For
the FDS6868A dual MOSFET in the demo circuit the
equivalent RDSON at 125C is (2*14mΩ) =28mΩ.
Thus if the current is greater than 0.21V/28mΩ =
7.5A the FAULT LED will turn on. Note that the
STATUS LED will also be on in this state.
from VCC1 and VCC2. FAULT derives its drive from
the greater of input voltage (V1, V2) or OUT. It becomes active when either voltage is greater than 0.9V.
The gate drive consists of a high current, wide bandwidth amplifier. When the amplifier is enabled, it attempts to regulate the GATE voltage such that the
voltage across the MOSFETs is approximately 15mV.
If the MOSFETs on resistance is so high as to prevent
regulation, then GATE goes to compliance and the
MOSFETs fully turn on. The inputs to the amplifier are
V1 or V2 and OUT.
The LT4351 gate drives derive their power from onchip boost regulators (L1, D3, C5 and L2, D8, C9).
These regulators will generate a VDD voltage approximately 10.7V above VIN.
If an external supply is available to supply VDD it can
be connected to VDD1 and VDD2. The on-chip regulators can be disabled by removing L1/D3 and L2/D8.
The LEDs take their power from the input however R2
and R8 can be removed and power can be supplied
QUICK START PROCEDURE
Demonstration circuit 694 is easy to set up to evaluate the performance of the LT4351. Refer to Figure 1
for proper measurement equipment setup and follow
the procedure below:
1.
With power off, connect the input power supplies
to V1, V2 and V3 and the respective GND’s.
2.
Set the output load to approximately 1A (12Ω at
12V)
3.
Turn on the V1 power and ramp up the voltage to
5V. The red FAULT LED should be on and green
STATUS LED off.
4.
The boost regulator will start going at VIN =0.9V.
The boost voltage will increase with increasing V1.
Check that the boost regulator is working by measuring the voltage on VDD1. With V1=5V, VDD1
should be approximately 15.7V
5.
Continue to turn up the V1 supply. At approximately 10V the red FAULT LED will go out and the
green STATUS LED will turn ON. Turn up the supply to 12V
6.
Monitor the OUT voltage. The load ammeter should
read 1A. The output voltage should be approximately 12V-1A*(2*10mΩ) =11.98V. When the load
current is reduced below 0.75A the OUT voltage
should hold at approximately 11.985. Turn the load
current back up to 1A.
7.
Check that the boost regulator is working by
measuring the voltage on VDD1. It should be approximately 22.7V
8.
Turn down the V1 supply until the red FAULT LED
turns on. This is the undervoltage trip point (approximately 9.6V).
9.
Turn up the V1 supply. At approximately 10V the
red FAULT LED will go out and the green STATUS
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 694
ORED POWER SUPPLY DEMO CIRCUIT
LED will turn ON. Turn up the supply till the red
FAULT LED turns on. This is the overvoltage trip
point (approximately 15V)
10. Turn
down the V1 supply and repeat steps 3 to 10
using the V2 supply. VDD2 is the boost regulator
output for the second channel.
11. The
third channel is a simple Schottky diode. With
V1 and V2 off, turn on V3 supply. Ramp up V3.
OUT should follow at approximately 0.3V below V3.
12. Turn
off V3. Turn on V1 and set to 12V. The green
STATUS LED on channel 1 should be ON. Turn on
V2 and raise the voltage. Initially the red FAULT
LED on channel 2 turns on and will stay on until V2
reaches 10V. At 10V that LED turns off. The green
STATUS LED on channel 2 will turn on when V2
reaches the V1 voltage. As you increase the V2
voltage the green STATUS LED on channel 1 will go
out.
13. You
can adjust V1 and V2 and load current to see
how the ORing function behaves. Current sharing
between channels will depend on the input voltages
V1
+
Power
Supply
20V/5A
V2
15. Optionally
Q3 and/or Q6 can be removed and individual SO8 MOSFETs can be used on the backside
of the board (Q1/Q2, Q4/Q5). This will allow the
user to use MOSFETs with lower RDSON for higher
current evaluation.
NOTE: The demo circuit has modest input and output
capacitance (10uF ceramic). Depending on your application more capacitance may be required.
When the current is abruptly turned off, inductance
on the input feed lines may ring above the maximum
voltage ratings for the MOSFETs. Keeping the inductance low and/or adding larger input capacitance with
appropriate ESR can help reduce this. Alternately, a
zener or voltage clamp can be added
+
+
+
-
V3
+
Power
20V/5A
desired the V3 supply can be turned on and used
for comparing the ORing between the LT4351 circuit and a Schottky diode. Note that the Schottky
forward drop is higher than the MOSFETs forward
drop so V3 will have to be set 0.3V or more higher
than V1 and V2.
+
-
20V/5A
Supply
14. If
V
Power
Supply
and the on resistances of the MOSFET pass elements.
-
V
+
-
A
-
Variable
Load
(100W)
V
-
+
V
-
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 694
ORED POWER SUPPLY DEMO CIRCUIT
4
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