ISL8270MEVAL1Z datasheet(16/56 Pages) RENESAS

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ISL8270M

FN8635 Rev.4.00

Page 16 of 56

Aug 17, 2017

Functional Description

SMBus Communications

The ISL8270M provides an SMBus digital interface that enables

the user to configure all aspects of the module operation as well

as monitor the input and output parameters. The ISL8270M can

be used with any SMBus host device. In addition, the module is

compatible with PMBus Power System Management Protocol

Specification Parts I & II version 1.2. The ISL8270M accepts most

standard PMBus commands. When controlling the device with

PMBus commands, it is recommended that the enable pin be

tied to SGND.

The SMBus device address is the only parameter that must be

set by external pins. All other device parameters can be set with

PMBus commands.

The ISL8270M can operate without the PMBus in pin-strap mode

with configurations programmed by pin-strap resistors, such as

output voltage, switching frequency, device SMBus address, input

UVLO, soft-start/stop, and current sharing. Note that pin-strap

resistors with 1% tolerance or better should be used for all the

pin-strap settings.

Output Voltage Selection

The output voltage can be set to a voltage between 0.6V and 5V

provided that the input voltage is higher than the desired output

voltage by an amount sufficient to maintain regulation.

The VSET pin is used to set the output voltage to levels as shown

in Table 4. The RSET resistor is placed between the VSET pin and

SGND. A standard 1% resistor is recommend.

The output voltage can also be set to any value between 0.6V and

5V using a PMBus command VOUT_COMMAND.

By default, VOUT_MAX is set 110% higher than VOUT set by the

pin-strap resistor, which can be changed to any value up to 5.5V

with PMBus command VOUT_MAX.

Soft-Start Delay and Ramp Times

It may be necessary to set a delay from when an enable signal is

received until the output voltage starts to ramp to its target

value. In addition, the designer may want to precisely set the

time required for VOUT to ramp to its target value after the delay

period has expired. These features can be used as part of an

overall inrush current management strategy or to precisely

control how fast a load IC is turned on. The ISL8270M gives the

system designer several options for precisely and independently

controlling both the delay and ramp time periods. The soft-start

delay period begins when the EN pin is asserted and ends when

the delay time expires.

The soft-start delay and ramp times can be programmed to

custom values with PMBus commands TON_DELAY and

TON_RISE. When the delay time is set to 0ms, the device begins its

ramp-up after the internal circuitry has initialized (approximately

2ms). When the soft-start ramp period is set to 0ms, the output

ramps up as quickly as the output load capacitance and loop

settings allow. It is generally recommended to set the soft-start

ramp to a value greater than 500µs to prevent inadvertent fault

conditions due to excessive inrush current.

Power-Good

The ISL8270M provides a Power-Good (PG) signal that indicates

the output voltage is within a specified tolerance of its target

level and no fault condition exists. By default, the PG pin asserts

if the output is within 10% of the target voltage. These limits and

the polarity of the pin can be changed with PMBus command

POWER_GOOD_ON.

A PG delay period is defined as the time from when all conditions

within the ISL8270M for asserting PG are met to when the PG pin

is actually asserted. This feature is commonly used instead of an

external reset controller to control external digital logic. A PG

delay can be programmed with PMBus command

POWER_GOOD_DELAY.

TABLE 4. OUTPUT VOLTAGE RESISTOR SETTINGS

VOUT

(V)

RSET

(kΩ)

0.60

0.65

0.70

12.1

0.75

13.3

0.80

14.7

0.85

16.2

0.90

17.8

0.95

19.6

1.00

21.5, or connect to SGND

1.05

23.7

1.10

26.1

1.15

28.7

1.20

31.6, or OPEN

1.25

34.8

1.30

38.3

1.40

42.2

1.50

46.4

1.60

51.1

1.70

56.2

1.80

61.9

1.90

68.1

2.00

2.10

82.5

2.20

90.9

2.30

100

2.50

110, or connect to V25

2.80

121

3.00

133

3.30

147

4.00

162

5.00

178

TABLE 4. OUTPUT VOLTAGE RESISTOR SETTINGS (Continued)

VOUT

(V)

RSET

(kΩ)

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