MAX1887 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX1887

Quick-PWM Slave Controllers for Multiphase, Step-Down Supplies

Maxim Integrated 

Quick-PWM Slave Controllers for

Multiphase, Step-Down Supplies

current-limit threshold of 40mV to 150mV. Use 1% toler-

ance resistors in the divider with 10µA to 20µA DC bias

current to prevent significant errors due to the ILIM

pin’s input current. Reducing the current-limit threshold

voltage lowers the sense resistor’s power dissipation,

but this also increases the relative measurement error:

VILIM(SLAVE)

20µA

≤ RD

≤

VILIM(SLAVE)

10µA

RC

=





VREF(MASTER)

VILIM(SLAVE)









− 1RD

Now, set the current-limit adjustment ratio (AADJ =

VITHM(HIGH)/VITHM(LOW)) greater than the maximum to

minimum on-resistance ratio (ARDS = RDS(ON)(MAX)/

RDS(ON)(MIN)):

AADJ ≥ AROS

1+





RA //RB

RLIMIT





≥

RDS(ON)(MAX)

RDS(ON)(MIN)

Increasing AADJ improves the master’s current-limit

accuracy but also increases the current limit’s noise

sensitivity. Therefore, RLIMIT may be selected using the

following equation:

( ) RLIMIT

≤

RA //RB RDS(ON)(MIN)

RDS(ON)(MAX) − RDS(ON)(MIN)

Finally, verify that the total load on the master’s refer-

ence does not exceed 50µA:

( ) IBIAS(TOTAL)



=





RA

+

VREF

RB // RLIMIT







+





VREF

RC + RD





≤ 50µA

Current Limit Design Example

For the typical application circuit shown in Figure 1: VIN

= 12V, VOUT = 1.3V, fSW = 300kHz, η = 2, ILOAD(MAX)

= 50A, L = 0.6µH, RDS(ON)(MAX) = 6mΩ, RDS(ON)(MIN )

= 3mΩ

1) Determine the peak-to-peak inductor current and

the valley current limit:

1.3V x(12V −1.3V)

∆IINDUCTOR = 12V x300kHz x0.6µH = 6.4A

ILIMIT(VALLEY)

=





50A

2





−





1

2

x

6.4A

=

21.8A

2) Determine the master’s current-limit threshold from

the valley current limit and low-side MOSFETs’ max-

imum on-resistance over temperature:

VITH(MASTER) ≥ 21.8A ✕ 6mΩ = 130mV

Now select the resistive-divider values (RA and RB

in Figure 5) to set the appropriate voltage at the

master’s ILIM input:

RB

=





10

x130mV

20µA





to





10 x130mV

10µA





=

65kΩ

to130kΩ

Selecting RB = 100kΩ ±1% provides the following

value for RA:

RA

=

 2V

 10x130mV



− 1

x100kΩ ≈54kΩ

3) Determine the slave’s current-limit threshold:

VITHS

≥

1.5mΩ

x





130mV

6mΩ

+

6.4 A

≈

42mV

Select the resistive-divider values (RC and RD in

Figure 5) to set the appropriate voltage at the

slave’s ILIM input:

RD

=

 10 x42mV 

 20µA 

 10 x42mV 

to 10µA 

=

21kΩ to42kΩ

Selecting RD = 30.1kΩ ±1% provides the following

value for RA:

RC

=

 2V

 10x42mV



− 1

x30.1kΩ ≈113kΩ

4) Determine RLIMIT (Figure 5) from the above equation:

(53.6kΩ //100kΩ) x 3mΩ

RLIMIT ≤

6mΩ − 3mΩ

≈ 35kΩ

5) Finally, verify that that the total bias currents do not

exceed the 50µA maximum load of the master’s ref-

erence:

( ) IBIAS(TOTAL)



=





54kΩ

+

2V

100kΩ // 34.8kΩ







+





2V

30.1kΩ +113kΩ





=

36µA

______________________________________________________________________________________ 21

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