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ISL8010

Monolithic 600mA Step-Down Regulator with Low Quiescent Current

Intersil 

ISL8010

Applications Information

Product Description

The ISL8010 is a synchronous, integrated FET 600mA step-

down regulator which operates from an input of 2.5V to 5.5V.

The output voltage is user-adjustable with a pair of external

resistors.

When the load is very light, the regulator automatically

operates in the PFM mode, thus achieving high efficiency at

light load (>70% for 1mA load). When the load increases,

the regulator automatically switches over to a voltage-mode

PWM operating at nominal 1.4MHz switching frequency. The

efficiency is up to 95%.

It can also operate in a fixed PWM mode or be synchronized

to an external clock up to 12MHz for improved EMI

performance.

PFM Operation

The heart of the ISL8010 regulator is the automatic

PFM/PWM controller.

If the SYNC pin is connected to ground, the regulator

operates automatically in either the PFM or PWM mode,

depending on load. When the SYNC pin is connected to VIN,

the regulator operates in the fixed PWM mode. When the pin

is connected to an external clock ranging from 1.6MHz to

12MHz, the regulator is in the fixed PWM mode and

synchronized to the external clock frequency.

In the automatic PFM/PWM operation, when the load is light,

the regulator operates in the PFM mode to achieve high

efficiency. The top P-channel MOSFET is turned on first. The

inductor current increases linearly to a preset value before it

is turned off. Then the bottom N-channel MOSFET turns on,

and the inductor current linearly decreases to zero current.

The N-channel MOSFET is then turned off, and an anti-

ringing MOSFET is turned on to clamp the LX pin to VO.

Both MOSFETs remain off until VFB drops below the internal

reference voltage of 0.8V. The inductor current looks like

triangular pulses. The frequency of the pulses is mainly a

function of output current. The higher the load, the higher the

frequency of the pulses until the inductor current becomes

continuous. At this point, the controller automatically

changes to PWM operation.

PWM Operation

The regulator operates the same way in the forced PWM or

synchronized PWM mode. In this mode, the inductor current

is always continuous and does not stay at zero.

In this mode, the P-channel MOSFET and N-channel

MOSFET always operate complementary. When the

P-channel MOSFET is on and the N-channel MOSFET off,

the inductor current increases linearly. The input energy is

transferred to the output and also stored in the inductor.

When the P-channel MOSFET is off and the N-channel

MOSFET on, the inductor current decreases linearly, and

energy is transferred from the inductor to the output. Hence,

the average current through the inductor is the output

current. Since the inductor and the output capacitor act as a

low pass filter, the duty cycle ratio is approximately equal to

VO divided by VIN.

The output LC filter has a second order effect. To maintain

the stability of the converter, the overall controller must be

compensated. This is done with the fixed internally

compensated error amplifier and the PWM compensator.

Because the compensations are fixed, the values of input

and output capacitors are 10µF to 22µF ceramic and

inductor is 1.5µH to 2.2µH.

Forced PWM Mode/SYNC Input

Pulling the SYNC pin HI (>2.5V) forces the converter into

PWM mode in the next switching cycle regardless of output

current. The duration of the transition varies depending on the

output current. Figures 22 and 23 (under two different loading

conditions) show the device goes from PFM to PWM mode.

Start-Up and Shutdown

When the EN pin is tied to VIN, and VIN reaches

approximately 2.4V, the regulator begins to switch. The

inductor current limit is gradually increased to ensure proper

soft-start operation.

When the EN pin is connected to a logic low, the ISL8010 is

in the shutdown mode. All the control circuitry and both

MOSFETs are off, and VOUT falls to zero. In this mode, the

total input current is less than 1µA.

When the EN reaches logic HI, the regulator repeats the

start-up procedure, including the soft-start function.

Current Limit and Short-Circuit Protection

The current limit is set at about 1.2A for the PMOS. When a

short-circuit occurs in the load, the preset current limit

restricts the amount of current available to the output, which

causes the output voltage to drop as load demand

increases. When the output voltage drops 30mV below the

reference voltage, the converter will shutdown for a period of

time, approximated by Equation 1, and then restart. If the

overcurrent condition still exists, it will repeat the shutdown-

wait-restart event. This is called a “hiccup” event. The

average power dissipation is reduced, thereby reducing the

likelihood of damage current and thermal conditions in the IC.

tHICCUP

≈





7----0---0----u-----⋅---V----I--N---

3

+

216 u

(EQ. 1)

Thermal Shutdown

Once the junction reaches about 145°C, the regulator shuts

down. Both the P-channel and the N-channel MOSFETs turn

off. The output voltage will drop to zero. With the output

MOSFETs turned off, the regulator will cool down. Once the

junction temperature drops to about 130°C, the regulator will

perform a normal restart.

10

FN6191.0

September 27, 2005

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