ISL8009A データシートの表示（PDF） - Renesas Electronics

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ISL8009A

1.5A Low Quiescent Current 1.6MHz High Efficiency Synchronous Buck Regulator

Renesas Electronics 

ISL8009A

Once the skip mode is entered, the pulse modulation starts being

controlled by the SKIP comparator shown in Figure 25. 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

20% of the current limit value (0.2V at the CSA output). 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 and 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.

Mode Control

The ISL8009A has a SKIP pin that controls the operation mode.

When the SKIP pin is driven to low or shorted to ground, the

regulator operates in a forced PWM mode. The forced PWM

mode remains the fixed PWM frequency at light load instead of

entering the skip mode.

Overcurrent Protection

The overcurrent protection is realized by monitoring the CSA

output with the OCP comparator, as shown in Figure 25. The

current sensing circuit has a gain of 0.4V/A, from the N-MOSFET

current to the CSA output. When the CSA output reaches 0.8V,

(which is equivalent to 2A 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 VFB pin

voltage for output short-circuit protection. When the VFB 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.

RSI/POR Function

When powering up, the open-collector Power-On-Reset output

holds low for about 1ms after VO reaches the preset voltage.

When the active-HI reset signal RSI is issued, POR goes to low

immediately and holds for the same period of time after RSI

comes back to LOW. The output voltage is unaffected (Figure 28).

When the function is not used, connect RSI to ground and leave

the pull-up resistor, R1, open at the POR pin.

The POR output also serves as a 1ms delayed Power-Good signal

when the pull-up resistor, R1, is installed. The RSI pin needs to be

directly or indirectly through another resistor connected to

ground for this to function properly.

.

VO

MIN

RSI

25ns

2ms

2ms

POR

FIGURE 28. RSI AND POR TIMING DIAGRAM

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 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.

Power MOSFETs

The power MOSFETs are optimized for best efficiency. The

ON-resistance for the P-MOSFET is typically 120m and the

ON-resistance for the N-MOSFET is typically 110m.

Duty Cycle

The ISL8009A features duty cycle operation to maximize the

battery life. When the battery voltage drops to a level that the

ISL8009A can no longer maintain the regulation at the output,

the regulator completely turns on the P-MOSFET. The maximum

drop out voltage under the duty-cycle operation is the product of

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

Enable

The Enable (EN) input allows the user to control the turning on or off

of the regulator for purposes such as power-up sequencing. When

the regulator is enabled, there is typically a 600µs delay for waking

up the bandgap reference, then the soft-start-up begins. When the

regulator is disabled, the P-MOSFET and the N-MOSFET are turned

off immediately. The 100 soft discharge resistor from LX to

GDN is activated and pulls the output to 0V.

Thermal Shutdown

The ISL8009A has built-in thermal protection. When the internal

temperature reaches +140°C, the regulator is completely

shutdown. As the temperature drops to +120°C, the ISL8009A

resumes operation by stepping through a soft-start-up.

FN6656 Rev 3.00

February 28, 2013

Page 11 of 13

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