ISL8088(2009) データシートの表示（PDF） - Intersil

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ISL8088
(Rev.:2009)

Dual 800mA Low Quiescent Current 2.25MHz High Efficiency Synchronous Buck Regulator

Intersil 

ISL8088

Once the skip mode is entered, the pulse modulation

starts being controlled by the SKIP comparator shown in

the “Block Diagram” on page 13. Each pulse cycle is still

synchronized by the PWM clock. The P-MOSFET is turned

on at the clock and turned off when its current reaches

the threshold of 250mA. As the average inductor current

in each cycle is higher than the average current of the

load, the output voltage rises cycle over cycle. When the

output voltage reaches 1.5% above the nominal voltage,

the P-MOSFET is turned off immediately. Then the

inductor current is fully discharged to zero and stays at

zero. The output voltage reduces gradually due to the

load current discharging the output capacitor. When the

output voltage drops to the nominal voltage, the

P-MOSFET will be turned on again at the clock, repeating

the previous operations.

The regulator resumes normal PWM mode operation

when the output voltage drops 1.5% below the nominal

voltage.

Synchronization Control

The frequency of operation can be synchronized up to

4MHz by an external signal applied to the SYNC pin. The

falling edge on the SYNC triggered the rising edge of the

PWM ON pulse.

Overcurrent Protection

CSA1 and CSA2 is used to monitor output 1 and output 2

channels respectively. The overcurrent protection is

realized by monitoring the CSA_ output with the OCP

threshold logic, as shown in “Block Diagram” on page 13.

The current sensing circuit has a gain of 0.285V/A, from

the P-MOSFET current to the CSA_output. When the

CSA_ output reaches the threshold of 590mV, the OCP

comparator is tripped to turn off the P-MOSFET

immediately. The overcurrent function protects the

switching converter from a shorted output by

monitoring the current flowing through the upper

MOSFETs.

Upon detection of overcurrent condition, the upper

MOSFET will be immediately turned off and will not be

turned on again until the next switching cycle.

PG

The power-good signal, (PG) monitors both of the output

channels. When powering up, the open-collector

power-on-reset output holds low for about 1ms after VO1

and VO2 reaches the preset voltages. The PG output also

serves as a 1ms delayed Power-Good signal. If one of the

output is disabled, then PG only monitors the active

channels. There is an internal 1MΩ pull-up resistor.

EN1

0

0

1

1

TABLE 1. PG

PG1

PG2

EN2 INTERNAL INTERNAL PG

0

X

X

0

1

X

1

1

0

1

X

1

1

1

1

1

UVLO

When the input voltage is below the undervoltage lock

out (UVLO) threshold, the regulator is disabled.

Enable

The enable (EN1, EN2) input allows user to control the

turning on or off the regulator for purposes such as

power-up sequencing. The regulator is enabled, there is

typically a 600µs delay for waking up the bandgap

reference, then the soft start-up begins.

Soft-Start-Up

The soft-start-up eliminates the in-rush 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.

In force PWM mode, the IC will continue to start-up in

PFM mode to support pre-biased load applications.

Discharge Mode (Soft-Stop)

When a transition to shutdown mode occurs, or the

output undervoltage fault latch is set, the outputs

discharge to GND through an internal 100Ω switch.

Power MOSFETs

The power MOSFETs are optimize for best efficiency. The

ON-resistance for the P-MOSFET is typically 180mΩ and

the ON- resistance for the N-MOSFET is typical 180mΩ.

100% Duty Cycle

The ISL8088 features 100% duty cycle operation to

maximize the battery life. When the battery voltage

drops to a level that the ISL8088 can no longer maintain

the regulation at the output, the regulator completely

turns on the P-MOSFET. The maximum dropout voltage

under the 100% duty-cycle operation is the product of

the load current and the ON-resistance of the P-MOSFET.

Thermal Shut Down

The ISL8088 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 ISL8088 resumes operation by stepping

through a soft-start-up.

Applications Information

Output Inductor and Capacitor Selection

To consider steady state and transient operation,

ISL8088 typically uses a 2.2µH 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 applications, in order to decrease the

inductor current ripple and output voltage ripple, the

15

FN6858.0

September 21, 2009

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