MAX5924BEUB+ Datasheet MAX5925DEUB+, MAX5925AEUB+, MAX5924DEUB+ | P16-P18

MAX5924/MAX5925/MAX5926

1V to 13.2V, n-Channel Hot-Swap Controllers

Require No Sense Resistor

16 ______________________________________________________________________________________

To determine the proper circuit-breaker resistor value

use the following equation, which refers to Figure 12:

where I

TRIPSLOW

is the desired slow-comparator trip

current.

The fast-comparator trip current is determined by the

selected R

value and cannot be adjusted indepen-

dently. The fast-comparator trip current is given by:

SC_DET must be connected to OUT through the select-

ed R

when not using R

SENSE

Mode

When operating with R

SENSE

, calculate the circuit-

breaker threshold using the worst possible operating

conditions, and add a 20% overcurrent margin to the

maximum circuit current. For example, with a maximum

expected current of 2A, using a 20% margin, the cur-

rent for calculation is (2A) x (1.2) = 2.4A. The resulting

minimum circuit-breaker threshold is then a product of

this current and R

SENSE

= 0.06Ω, or (0.06Ω) x (2.4A) =

0.144V. Using this method to choose a false circuit-

breaker threshold allows the circuit to operate under

worst-case conditions without causing a circuit-breaker

fault, but the circuit-breaker function will still detect a

short-circuit or a gross overcurrent condition.

To determine the proper circuit-breaker resistor value,

use the following equation, which refers to Figure 13:

where, I

TRIPSLOW

is the desired slow-comparator trip

current.

The fast-comparator trip current is determined by the

selected R

value and cannot be adjusted indepen-

dently. The fast-comparator trip current is given by:

SC_DET should be connected to V

when using

SENSE

IxR R V

TRIPFAST

CBF

CB OS

SENSE

()

IxRV

TRIPSLOW SENSE

()

IxR R V

TRIPFAST

CBF

CB OS

DS ON

()

IxR V

TRIPSLOW DS ON

CB OS

()

SELECT

MAX5925

MAX5926

SENSE

GATE

LOAD

CBF

SENSE

OUT

SLOW

COMPARATOR

CB,TH

CB,OS

FAST

COMPARATOR

CBF,TH

CB,OS

Figure 13. Circuit Breaker Using R

SENSE

SELECT

SLOW

COMPARATOR

CB,TH

MAX5925

MAX5926

SENSE

GATE

LOAD

CBF

DS(ON)

CB,OS

OUT

FAST

COMPARATOR

CBF,TH

CB,OS

Figure 12. Circuit Breaker Using R

DS(ON)

MAX5924/MAX5925/MAX5926

1V to 13.2V, n-Channel Hot-Swap Controllers

Require No Sense Resistor

______________________________________________________________________________________ 17

Circuit-Breaker Temperature Coefficient

In applications where the external MOSFET’s on-resis-

tance is used as a sense resistor to determine overcur-

rent conditions, a 3300ppm/°C temperature coefficient

is desirable to compensate for the R

DS(ON)

tempera-

ture coefficient. Use the MAX5926’s TC input to select

the circuit-breaker programming current’s temperature

coefficient, TC

ICB

(see Table 2). The MAX5924 temper-

ature coefficient is preset to 0ppm/°C, and the

MAX5925’s is preset to 3300ppm/°C.

Setting TC

ICB

to 3300ppm/°C allows the circuit-breaker

threshold to track and compensate for the increase in the

MOSFET’s R

DS(ON)

with increasing temperature. Most

MOSFETs have a temperature coefficient within a

3000ppm/°C to 7000ppm/°C range. Refer to the MOSFET

data sheet for a device-specific temperature coefficent.

DS(ON)

and I

are temperature dependent, and can

therefore be expressed as functions of temperature. At

a given temperature, the MAX5925/MAX5926 indicate

an overcurrent condition when:

TRIPSLOW

x R

DS(ON)

(T) ≥ I

(T) x R

+ |V

CB,

where V

CB,OS

is the worst-case offset voltage. Figure 14

graphically portrays operating conditions for a MOSFET

with a 4500ppm/°C temperature coefficient.

Applications Information

Component Selection

n-Channel MOSFET

Most circuit component values may be calculated with

the aid of the MAX5924–MAX5926. The "Design calcula-

tor for choosing component values" software can be

downloaded from the MAX5924–MAX5926 Quickview on

the Maxim website.

Select the external n-channel MOSFET according to the

application’s current and voltage level. Table 3 lists some

recommended components. Choose the MOSFET’s

on-resistance, R

DS(ON)

, low enough to have a minimum

voltage drop at full load to limit the MOSFET power dis-

sipation. High R

DS(ON)

can cause undesired power

loss and output ripple if the board has pulsing loads or

triggers an external undervoltage reset monitor at full

load. Determine the device power-rating requirement to

accommodate a short circuit on the board at startup

with the device configured in autoretry mode

Using the MAX5924/MAX5925/MAX5926 in latched mode

allows the consideration of MOSFETs with higher R

DS(ON)

and lower power ratings. A MOSFET can typically with-

stand single-shot pulses with higher dissipation than the

specified package rating. Low MOSFET gate capaci-

tance is not necessary since the inrush current limiting is

achieved by limiting the gate dv/dt. Table 4 lists some

recommended manufacturers and components.

Be sure to select a MOSFET with an appropriate gate

drive (see the

Typical Operating Characteristics

Typically, for V

less than 3V, select a 2.5V V

MOSFET.

Table 2. Programming the Temperature

Coefficient (MAX5926)

TC TC

ICB

(ppm/°C)

High 0

Low 3300

Table 3. Suggested External MOSFETs

APPLICATION

CURRENT (A)

PART DESCRIPTION

International Rectifier

IRF7401

SO-8

2 Siliconix Si4378DY SO-8

5 Siliconix SUD40N02-06 DPAK

10 Siliconix SUB85N02-03 D2PAK

TEMPERATURE (°C)

AND V

SENSE

(mV)

85603510-15

-40 110

= V

= 13.2V, R

= 672Ω, I

TRIPSLOW

= 5A,

DS(ON)

(25) = 6.5mΩ

CIRCUIT-BREAKER TRIP REGION

SENSE

≥ V

)

= I

(T) x R

+ V

CB,OS

(3300ppm/°C)

SENSE

= R

DS(ON)

(T) x I

LOAD(MAX)

(4500ppm/°C)

Figure 14. Circuit-Breaker Trip Point and Current-Sense

Voltage vs. Temperature

MAX5924/MAX5925/MAX5926

1V to 13.2V, n-Channel Hot-Swap Controllers

Require No Sense Resistor

18 ______________________________________________________________________________________

Optional Sense Resistor

Select the sense resistor in conjunction with R

to set

the slow and fast circuit-breaker thresholds (see the

Selecting a Circuit-Breaker Threshold

section). The

sense-resistor power dissipation depends on the device

configuration. If latched mode is selected, P

RSENSE

OVERLOAD

)

x R

SENSE

; if autoretry is selected, then

RSENSE

= (I

OVERLOAD

)

x R

SENSE

x (t

RETRY

Choose a sense-resistor power rating of twice the

RSENSE

for long-term reliable operation. In addition,

ensure that the sense resistor has an adequate I

T rating

to survive instantaneous short-circuit conditions.

No-Load Operation

The internal circuitry is capable of sourcing a current at

the OUT terminal of up to 120µA from a voltage V

. If there is no load on the circuit, the output capacitor

will charge to a voltage above V

until the external MOS-

FET’s body diode conducts to clamp the capacitor volt-

age at VIN plus the body-diode V

. When testing or

operating with no load, it is therefore recommended that

the output capacitor be paralleled with a resistor of value:

R = V

/ 120µA

where V

is the maximum acceptable output voltage

prior to hot-swap completion.

Design Procedure

Given:

• V

= V

= 5V

• C

= 150µF

• Full-Load Current = 5A

• No R

SENSE

• I

INRUSH

= 500mA

Procedures:

1) Calculate the required slew rate and corresponding

SLEW

2) Select a MOSFET and determine the worst-case

power dissipation.

3) Minimize power dissipation at full load current and

at high temperature by selecting a MOSFET with an

appropriate R

DS(ON)

. Assume a 20°C temperature

difference between the MAX5924/MAX5925/

MAX5926 and the MOSFET.

For example, at room temperature the IRF7822’s

DS(ON)

= 6.5mΩ. The temperature coefficient for

this device is 4000ppm/°C. The maximum R

DS(ON)

for the MOSFET at T

J(MOSFET)

= +105°C is:

The power dissipation in the MOSFET at full load is:

4) Select R

Since the MOSFET’s temperature coefficient is

4000ppm/°C, which is greater than TC

ICB

(3300ppm/°C), calculate the circuit-breaker thresh-

old at high temperature so the circuit breaker is

guaranteed not to trip at lower temperature during

normal operation (Figure 15).

TRIPSLOW

= I

FULL LOAD

+ 20% = 5A + 20% = 6A

DS(ON)105

= 8.58mΩ (max), from step 2

CB85

= 58µA x (1 + (3300ppm/°C x (85 - 25)°C)

= 69.5µA (min)

= ((6A x 8.58mΩ) + 4.7mV)/69.5µA = 808Ω

IxR V

TRIPSLOW

DS ON CB OS

()

() ,105

PIR A m mW

== × Ω=

5 8 58 215() .

RmCC

ppm

DS ON()

. ( )

105

6 5 1 105 25 4000

858

=×+°−°×

⎛

⎝

⎜

⎞

⎠

⎟

=Ω

Ω

SLEW

−−

330 10 330 10

. µ

INRUSH

1000

Table 4. Component Manufacturers

COMPONENT MANUFACTURER PHONE WEBSITE

Dale-Vishay 402-564-3131 www.vishay.com

Sense Resistors

IRC 828-264-8861 www.irctt.com

Fairchild 888-522-5372 www.fairchildsemi.com

MOSFETs

International Rectifier 310-233-3331 www.irf.com

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21

MAX5924BEUB+

Mfr. #:

Buy MAX5924BEUB+

Manufacturer:

Maxim Integrated

Description:

Hot Swap Voltage Controllers 1-13.2V Hot-Swap Controller

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MAX5924BEUB+

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