ISL9111, ISL9111A

8

FN7602.3

December 7, 2011

Detailed Description

Current Mode PWM Operation

The control scheme of the device is based on the peak current

mode control, and the control loop is compensated internally.

The peak current of the N-channel MOSFET switch is sensed to

limit the maximum current flowing through the switch and the

inductor. The typical current limit is set to 1A.

The control circuit includes ramp generator, slope compensator,

error amplifier, PWM comparator (see block diagrams on page 2

and page 3). The ramp signal is derived from the inductor

current. This ramp signal is then compared to the error amplifier

output to generate the PWM gating signals for driving both

N-channel and P-channel MOSFETs. The PWM operation is

initialized by the clock from the internal oscillator (typical

1.2MHz). The N-channel MOSFET is turned on at the beginning of

a PWM cycle, the P-channel MOSFET remains off, and the current

starts ramping up. When the sum of the ramp and the slope

compensator output reaches the error amplifier output voltage,

the PWM comparator outputs a signal to turn off the N-channel

MOSFET. Here, both MOSFETs remain off during the dead-time

interval, and then the P-channel MOSFET is turned on and

remains on until the end of this PWM cycle. During this time, the

inductor current ramps down until the next clock. At this point,

following a short dead time, the N-channel MOSFET is again

turned on, repeating as previously described.

Skip Mode Operation

The device is capable of operating in two different modes. When

the inductor current is sensed to cross zero for eight consecutive

times, the converter enters skip mode. In skip mode, each pulse

cycle is still synchronized by the PWM clock. The N-channel

MOSFET is turned on at the rising edge of the clock and turned

off when the inductor peak current reaches typically 25% of the

current limit. Then the P-channel MOSFET is turned on, and it

stays on until its current goes to zero. Subsequently, both

N-channel and P-channel MOSFETs are turned off until the next

clock cycle starts, at which time the N-channel MOSFET is turned

output voltage, the N-channel MOSFET is immediately turned off

and the P-channel MOSFET is turned on until the inductor current

goes to zero. The N-channel MOSFET resumes operation when

operation. The converter returns to 1.2MHz PWM mode

Given the skip mode algorithm incorporated in the ISL9111 and

ISL9111A, the average value of the output voltage is

approximately 0.75% higher than the nominal output voltage

under PWM operation. This positive offset improves the load

transient response when switching from skip mode to PWM

mode operation. The ripple on the output voltage is typically

output voltage, and it increases as input voltage approaches

output voltage. Figure 9 shows the ripple voltage versus input

voltage.

Synchronous Rectifier

The ISL9111 and ISL9111A integrate one N-channel MOSFET

and one P-channel MOSFET to realize synchronous boost

converters. Because the commonly used discrete Schottky

power conversion efficiency reaches a value above 90%. Since a

typical step-up converter has a conduction path from the input to

the output via the body diode of the P-channel MOSFET, a special

circuit (see Block Diagrams on page 2 and page 3) is used to

reverse the polarity of the P-channel body diode when the part is

shut down. This configuration completely disconnects the load

from the input during shutdown of the converter. The benefit of

this feature is that the battery will not be completely depleted

during shutdown of the converter. No additional components are

needed to disconnect the battery from the output of the

converter.

Minimum Startup and Minimum Operating

Voltage

The ISL9111 and ISL9111A address applications with

rechargeable and non-rechargeable battery cells.

1. The ISL9111 has a UVLO feature. The part can start up with a

0.8V typical input voltage and internally shut off when the

battery drops below 0.7V to protect a rechargeable battery

from over-discharge.

FIGURE 5A. POWER SUPPLY SOLUTION FOR 5.25V OUTPUT VOLTAGE

SW

EN

GND

FB

VOUT

VIN

6

5

4

3

2

1

4.7µH

4.7µF

4.7µF

923k

166k

5.25V

ISL9111, ISL9111A - ADJ

40

50

60

70

80

90

100

0.1

1.0

10

100

1000