ISL9506 datasheet(21/28 Pages) RENESAS

21 / 28 page

ISL9506

FN6722 Rev 0.00

Page 21 of 28

August 13, 2008

As mentioned in the ““Theory of Operation” on page 16

section of this data sheet, PGD_N is logic level high at start-

up. When the output voltage reaches 90% of start voltage, a

counter is enabled, it counts 13 switching cycles (about 43µs

for 300kHz operation) then PGD_N goes low. This in turn

triggers an internal timer for the PGOOD signal. This timer

allows PGOOD to go high approximately 7ms after PGD_N

goes low.

Static Mode of Operation - Remote Voltage

Sensing

Remote Voltage sensing allows the Voltage Regulator to

compensate for various resistive drops in the power path

and insure that the voltage seen at the point of load is the

correct level independent of load current.

The VSEN and RTN of the ISL9506 are the Kelvin

connection pins to the point of load. This allows the Voltage

Regulator to tightly control the output voltage at the point of

load, independent of layout inconsistencies and drops. This

Kelvin sense technique provides for extremely tight droop

impedance regulation.

These traces should be laid out as noise sensitive traces.

For optimum droop impedance regulation performance, the

traces connecting these two pins to the Kelvin sense point of

the load must be laid out in parallel and away from rapidly

rising voltage nodes (switching nodes) and other noisy

traces. To achieve optimum performance, place common

mode and differential mode RC filters to analog ground on

VSEN and RTN as shown in Figure 46. However, the filter

resistors should be in order of 10

so that they do not

interact with the 50k

 input resistance of the differential

amplifier.

Due to the fact that the voltage feedback to the switching

regulator is sensed at the point of load, there exists the

potential of an overvoltage due to an open circuit feedback

signal, should the regulator be operated without the load

installed. Due to this fact, we recommend the use of the

ROPN1 and ROPN2 connected to VOUT and ground as

shown in Figure 46. These resistors will provide voltage

feedback in the event that the system is powered up without

the load installed. These resistors are typically 100

.

Setting the Switching Frequency - FSET

The R3 modulator scheme is not a fixed frequency PWM

architecture. The switching frequency can increase during

the application of a load to improve transient performance.

However, it also varies slightly due changes in input and

output voltage and output current, but this variation is

normally less than 10% in continuous conduction mode.

Refer to Figure 39. A resistor connected between the VW

and COMP pins of the ISL9506 adjusts the switching

window, and therefore adjusts the switching frequency. The

RFSET resistor that sets up the switching frequency of the

converter operating in CCM can be determined using the

following relationship, where RFSET is in k and the

switching period is in µs.

In discontinuous conduction mode, (DCM), the ISL9506 runs

in period stretching mode. It should be noted that the

switching frequency in the Electrical Specification Table is

tested with the error amplifier output or Comp pin voltage at

2V. When Comp pin voltage is lower, the switching

frequency will not be at the tested value but can still maintain

the output voltage ripple within specification.

Voltage Regulator Thermal Throttling

The ISL9506 features a thermal monitor which senses the

voltage change across an externally placed negative

temperature coefficient (NTC) thermistor, see Figure 45.

Proper selection and placement of the NTC thermistor

allows for detection of a designated temperature rise by the

system.

Figure 45 shows the thermal throttling feature with

hysteresis. At low temperature, SW1 is on and SW2

connects to the 1.20V side. The total current going from NTC

pin is 60µA. The voltage on NTC pin is higher than threshold

voltage of 1.20V and the comparator output is low. VRHOT#

is pulling up high by the external resistor.

When temperature increases, the NTC thermistor resistance

on NTC pin decreases. The voltage on NTC pin decreases

to a level lower than 1.20V. The comparator changes polarity

and turns SW1 off and throws SW2 to 1.24V. This pulls

VRHOT# low and sends the signal to start thermal throttle.

There is a 6µA current reduction on NTC pin and 40mV

voltage increase on threshold voltage of the comparator in

this state. The VRHOT# signal will be used to change the

load operation and decrease the power consumption. When

the temperature goes down, the NTC thermistor voltage will

eventually go up. If NTC voltage increases to 1.24V, the

comparator will then be able to flip back. The external

Rfset k





Period

s

 0.29

–

 2.33



(EQ. 6)

NTC

RNTC

VNTC

VRHOT#

1.24V

54µA

INTERNAL TO

ISL9506

FIGURE 45. CIRCUITRY ASSOCIATED WITH THE THERMAL

THROTTLING FEATURE OF THE ISL9506

6µA

1.20V

SW1

SW2

ISL9506 Datasheet(PDF) 21 Page - Renesas Technology Corp

ISL9506 Datasheet(HTML) 21 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL