ISL8011IRZ Datasheet ISL8011IRZ-T, ISL8011IRZ | P7-P9

FN9254.2

August 4, 2009

Theory of Operation

ISL8011 is an integrated FET, 1.2A synchronous buck

regulator for general purpose point-of load applications. The

regulator operates at 1.5MHz fixed switching frequency

under heavy load condition to allow small external inductor

and capacitors to be used for minimal printed-circuit board

(PCB) area. The supply current is typically only 0.1µA when

the regulator is shut down.

PWM Control Scheme

The ISL8011 employs the current-mode pulse-width

modulation (PWM) control scheme for fast transient

response and pulse-by-pulse current limiting. Figure 11

shows the block diagram. The current loop consists of the

oscillator, the PWM comparator COMP, current sensing

circuit, and the slope compensation for the current loop

stability. The current sensing circuit consists of the

resistance of the P-Channel MOSFET when it is turned on

and the current sense amplifier (CSA1). The gain for the

current sensing circuit is typically 0.4V/A. The control

reference for the current loops comes from the error

amplifier EAMP of the voltage loop.

The PWM operation is initialized by the clock from the

oscillator. The P-Channel MOSFET is turned on at the

beginning of a PWM cycle and the current in the MOSFET

starts to ramp up. When the sum of the CSA1 and the

compensation slope (0.675V/µs) reaches the control

reference of the current loop, the PWM comparator COMP

sends a signal to the PWM logic to turn off the P-MOSFET

and to turn on the N-Channel MOSFET. The N-MOSFET

stays on until the end of the PWM cycle. Figure 12 shows

the typical operating waveforms during the PWM operation.

The dotted lines illustrate the sum of the compensation ramp

and the CSA1 output.

The output voltage is regulated by controlling the reference

voltage to the current loop. The bandgap circuit outputs a

0.8V reference voltage to the voltage control loop. The

feedback signal comes from the FB pin. The soft-start block

only affects the operation during the start-up and will be

discussed separately shortly. The error amplifier is a

transconductance amplifier that converts the voltage error

signal to a current output. The voltage loop is internally

compensated with the 25pF and 400kΩ RC network. The

maximum EAMP voltage output is precisely clamped to the

bandgap voltage (1.172V).

Overcurrent Protection

The overcurrent protection is realized by monitoring the

CSA1 output with the OCP comparator, as shown in

Figure 11. The current sensing circuit has a gain of 0.4V/A,

from the N-MOSFET current to the CSA1 output. When the

CSA1 output reaches 1V, which is equivalent to 2.5A for the

switch current, the OCP comparator is tripped to turn off the

P-MOSFET immediately.

Short-Circuit Protection

A short-circuit protection (SCP) comparator monitors the FB

pin voltage for output short-circuit protection. When the FB is

lower than 0.2V, the SCP comparator forces the PWM

oscillator frequency to drop to 1/3 of the normal operation

value. This comparator is effective during start-up or an

output short-circuit event.

POR Signal

The ISL8011 offers a power-on reset (POR) signal for

resetting the microprocessor at the power-up. When the

output voltage is not within a power-good window, the POR

pin outputs an open-drain low signal to reset the

microprocessor. The output voltage is monitored through the

FB pin. When the voltage of the monitored node is within the

window of 0.736V and 0.864V, a power-good signal is issued

to turn off the open-drain POR pin. The rising edge of the

POR output is delayed by 200ms.

UVLO

When the input voltage is below the undervoltage lock out

(UVLO) threshold, the regulator is disabled.

Soft Start-Up

The soft start-up eliminates the inrush current during the

start-up. The soft-start block outputs a ramp reference to

both the voltage loop and the current loop. The two ramps

limit the inductor current rising speed as well as the output

voltage speed so that the output voltage rises in a controlled

fashion. At the very beginning of the start-up, the output

voltage is less than 0.2V; hence the PWM operating

frequency is 1/3 of the normal frequency. Figure 7 shows the

start-up waveforms.

Power MOSFETs

The power MOSFETs are optimized for best efficiency. The

ON-resistance for the P-MOSFET is typically 150mΩ and

the ON-resistance for the N-MOSFET is typically 150mΩ.

100% Duty Cycle

The ISL8011 features 100% duty cycle operation to

maximize the battery life. When the battery voltage drops to

FIGURE 12. PWM OPERATION WAVEFORMS

EAMP

CSA1

Duty

Cycle

OUT

ISL8011

FN9254.2

August 4, 2009

a level that the ISL8011 can no longer maintain the

regulation at the output, the regulator completely turns on

the P-MOSFET. The maximum drop out voltage under the

100% duty-cycle operation is the product of the load current

and the ON-resistance of the P-MOSFET.

Enable

The enable (EN) input allows user to control the turning on

or off the regulator for purposes such as power-up

sequencing. The the regulator is enabled, there is typically a

300µs delay for waking up the bandgap reference. Then the

soft start-up begins. When the regulator is disabled, the

P-MOSFET is turned off immediately and the N-MOSFET is

turned on.

Thermal Shut Down

The ISL8011 has built-in thermal protection. When the

internal temperature reaches +150°C, the regulator is

completely shut down. As the temperature drops to +130°C,

the ISL8011 resumes operation by stepping through a soft

start-up.

By-Passing

The V

is voltage is the supply to the internal control circuit

and is derived from the PVIN pin. An internal 5Ω resistor

connects the two pins and also serves as an filtering resistor.

An external 0.1µF ceramic capacitor is recommended to

by-pass the V

supply.

Applications Information

Output Inductor and Capacitor Selection

To consider state steady and transient operation, ISL8011

typically uses a 1.8µH output inductor. Higher or lower

inductor values can be used to optimize the total converter

system performance. For example, for higher output voltage

3.3V application, in order to decrease the inductor current

ripple and output voltage ripple, the output inductor value

can be increased as shown in Table 1. The inductor ripple

current can be expressed as shown in Equation 1:

The inductor’s saturation current rating needs be at least

larger than the peak current. The maximum peak current of

ISL8011 is 2.1A. The saturation current needs be over 2.1A

for maximum output current application.

ISL8011 uses internal compensation network and the output

capacitor value is dependent on the output voltage. The

ceramic capacitor is recommended to be X5R or X7R. The

recommended minimum output capacitor values are shown

in Table 1.

In Table 1, the minimum output capacitor value is given for

different output voltage to make sure the whole converter

system stable. Due to the limitation on power dissipation

when the regulator disable and discharge output capacitor,

there is the maximum output capacitor value. The maximum

output capacitor value is variable with the output voltage.

The plot curve is shown in Figure 13.

Input Capacitor Selection

The main functions for the input capacitor are to provide

decoupling of the parasitic inductance and to provide filtering

function to prevent the switching current flowing back to the

supply rail. A 10µF X5R or X7R ceramic capacitor is a good

starting point for the input capacitor selection.

Output Voltage Setting Resistor Selection

The resistors R

and R

shown in Figure 10 set the output

voltage for the adjustable version. The output voltage can be

calculated by using Equation 2:

where the 0.8V is the reference voltage. To minimize the

accuracy impact on the output voltage, select the R

and R

no larger than 100kΩ.

ΔI

---------

–

⎝⎠

⎜⎟

⎛⎞

•

-------------------------------------- -

(EQ. 1)

TABLE 1. OUTPUT CAPACITOR VALUE vs V

OUT

0.8V 10µF 1.0µH~2.2µH

1.2V 10µF 1.2µH~2.2µH

1.6V 10µF 1.8µH~2.2µH

1.8V 10µF 1.8µH~3.3µH

2.5V 10µF 1.8µH~3.3µH

3.3V 6.8µF 1.8µH~4.7µH

3.6V 4.7µF 1.8µH~4.7µH

600

505

410

315

220

125

OUTPUT CAPACITOR VALUE (µF)

1.270.8 1.73 2.2 2.67 3.13 3.6

OUTPUT VOLTAGE (V)

FIGURE 13. THE MAXIMUM CAP vs THE OUTPUT VOLTAGE

0.8 1

-------

⎝⎠

⎜⎟

⎛⎞

•=

(EQ. 2)

ISL8011

FN9254.2

August 4, 2009

Layout Recommendation

The layout is a very important converter design step to make

sure the designed converter works well. For ISL8011 buck

converter, the power loop is composed of the output inductor

L, the output capacitor C

OUT

, Phase pin and PGND pin. It is

necessary to make the power loop as small as possible. In

order to make the output voltage regulate well and avoid the

noise coupling from the power loop, SGND pin should be

connected with PGND pin at the terminals of the load.

The heat of the IC is mainly dissipated through the thermal

pad. Maximizing the copper area connected to the thermal

pad is preferable. In addition, a solid ground plane is helpful

for EMI performance.

ISL8011

P1-P3 P4-P6 P7-P9 P10-P10

ISL8011IRZ

Mfr. #:

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Manufacturer:

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Description:

Switching Voltage Regulators W/ANNEAL LW IQ BUCKG W/INTEGRTD FET

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