Linear Technology LTC2923 Bedienungsanleitung

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732

POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER

LTC2923

DESCRIPTION

Demonstration circuit 732 allows evaluating perform-

ance of the LTC2923 Power Supply Tracking Controller

which is intended to provide hot swapping of one not

regulated voltage source (master supply) and tracking

or sequencing of two other regulated power supplies

via current injection into their respective feedback

nodes.

The DC732 provides two different configurations on

one snap apart circuit board.

Circuit 1 provides for tracking of two regulated sup-

plies via feedback node current injection.

Circuit 2 provides an additional pass FET to control a

third fixed rail on a larger board. Both boards provide

very small footprints for easy installation into existing

systems.

Design files for this circuit board are available. Call

the LTC factory.

LTC is a trademark of

Linear

Technology Corporation

Table 1.

Performance Summary (T

= 25°C)

PARAMETER CONDITION VALUE

Supply Voltage Rated operating limits 2.9 • V

• 5.5V

Under voltage lockout V

rising 2.2V • V

• 2.7V

ON Pin Threshold Precision threshold accurately holds off power supplies until

both supplies are fully up 1.212 • V

VON(TH)

• 1.248V

ON Pin Hysteresis 30mV• ˜V

ON(HYS)

•150mV30A

Gate Pin Current

Gate On : V

GATE

=0V, No Faults

Gate Off: V

GATE

=5V, No Faults

Gate Fault: V

GATE

=5V, Short-Circuit Fault

-7µA •

GATE

• -13 µA

7 µA • I

GATE

• 13 µA

5mA • I

GATE

• 50mA

FET Drain-Source Overcurrent Voltage Threshold (V

RAMP

) FET Over Current Sensed by Drain-Source Voltage 160mV • V

DSTH

• 240mV

Fixed Rail Current Capability Limited by Si4412 MOSFET (To Control Higher Current Re-

place MOSFET with a Larger Device) 4A Max

Track Pin Voltage 0.776V • V

TRACK

• 0.824V

Feedback Pin Compliance Regulated Power Supplies Feedback Voltage Must be Less

Than 1.5V 0V • V

• 1.5V

Feedback Node Control Current V

• 1.5V 1V MIN

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732

POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER

OPERATING PRINCIPLES

The LTC2923 controls two regulated power supplies by

introducing a precision controlled current into the feed-

back node of the supplies.

A third fixed rail can be controlled via a series pass FET.

The power supply’s feedback node must be accessible

and should be high impedance.

NOTE:

The feedback nodes of many linear regulators

(and of some boost converters) do not meet the above

high input impedance requirement when the output volt-

age is programmed below•0.6V or V

REF

of the supply is

not suitable for tracking by feedback node current injec-

tion. Sequencing and tracking via series pass FET using

the LTC2921 is a preferred approach.

NOTE

: The LTC2923 V

must be present and above

CC(UVL)

BEFORE the tracked supplies are enable or pow-

ered up. If V

is not present, the slave supplies will

power up normally and uncontrolled.

The series pass FET can control a third primary supply

or a supply where the feedback node either is not acces-

sible or does not provide the required high impedance

such as a linear LDO. An electronic circuit breaker func-

tion implemented in the LTC2923 protects the pass FET.

This function is activated after the pass FET is fully en-

hanced. The LTD2923 monitors the drain to source volt-

age and rapidly pulls the gate voltage to ground if FET

drops out of saturation.

The LTC2923 is designed to track two slave supplies to

an externally generated ramp voltage applied to the

RAMP pin. This ramp voltage can be generated by using

the LTC2923 GATE pin constant current source to charge

a capacitor or using the output of the pass FET con-

trolled supply or using other voltage source is condi-

tioned not to exceed V

A pair of resistors configures the behavior of each slave

supply relative to the master ramp signal. The choice of

resistors can control a slave supply to track the master

ramp signal coincidently, ratiometrically, with offset and

or time delay (sequencing).

The high impedance feedback current outputs of the

LTC2923 do not load the feedback node of the slave

supplies and therefore do not affect the slave power

supply stability or transient response.

The LTC2923 is able to control power supplies satisfying

these two conditions:

The feedback node potential must be lower than

1.5V when the supply is in regulation.

The power supply feedback divider current

should satisfy the following expression:

10µA•V

•1mA. Where R

is the parallel (Theve-

nin) impedance of the feedback resistors and V

the reference voltage of the power supply.

Condition 1 requires the slave power supply feed-

back voltage to be within the compliance range of

the LTC2923 current source output.

Condition 2 requires the power supply feedback

node impedance to be within a range where a0 the

available control current can drive the slave power

supply output voltage to zero and b) the LTc2923

leakage current does not introduce errors in the

power supply output voltage exceeding ½% worst

case.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732

POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER

QUICK START PROCEDURE

Demonstration Circuit DC732 provides two separate cir-

cuits, one with pass FET capable of handling a 4A supply

and a smaller board without the pass FET. Two circuits

can be separated by snapping them apart along the

score line. The back sides of the boards contain no parts

to facilitate mounting the boards into an existing system

with double sides adhesive tape if needed.

The board with drawing number DC732A-1 does not

have a power FET and the master ramp signal is derived

from the LTC2923 gate pin voltage. Both boards sche-

matics are showh in Fig.2 and board assembly=ies in

Fig.2.

NOTE: Because of the many options and circuit depend-

encies on the slave supplies involved, DC732A is pro-

vided without RTA1, RTB1, RTA2, and RTB2 installed.

Refer to the LTC2923 Data Sheet available at

www.linear.com for extensive design instructions on

calculating the values of R

TAX

< R

TBX

as well as options for

ONA

, R

ONB

, and C

GATE.

Initial information for the LTC2923 adjustment is:

- master power supply (unregulated )voltage, steady

state load current and capacitive load,

- power supply feedback node resistors R

FA1

, R

FB1

, R

FA2

FB2,

- defined power supply output voltage profile in the

power-up and power-down,

- all supply output voltage slew rate a=dV/dt (a

MASTER

SLAVE_1

, a

SLAVE_2

- desired delay time (in the offset tracking or in supply

sequencing).

1) Chose power MOSFET for master supply and install

on the board (only for board DC732A).

2) Calculate the resistor R

FA1

, R

FB1

, R

FA2

, R

FB2

values and

gate pin capacitor value (C1 for DC732A board and C4

for DC732A-1 board) according to the LTC2923 Data

Sheet and install on the board.

3) Connect power supply feedback nodes with

FEEDBACK1 (TP4) and FEEDBACK2 (TP7) pins.

4) Connect power supply with board GND with TP8.

5) Connect load to MASTER OUTPUT VOLTAGE (TP9).

6) Apply MASTER INPUT VOLTAGE and observe tran-

sient.

Note that that

circuit resistors R

ONA

and R

ONB

are

configured for a 2.9V turn-on signal level.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732

POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732

POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER

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Authorized Distributor

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DC732A

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