ISL9113A

FN8620 Rev 1.00

Page 5 of 10

January 4, 2017

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 1.3A.

The control circuit includes a ramp generator, slope

compensator, error amplifier, and a PWM comparator (see

“Block Diagram” on page 2). 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

1.8MHz). 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. Next, 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 boost converter 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

than the nominal 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

repeating the previous operation. The converter returns to

nominal voltage.

Given the Skip mode algorithm incorporated in the ISL9113A,

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

voltage is sufficiently lower than output voltage, and it increases

as the input voltage approaches the output voltage.

Synchronous Rectifier

The ISL9113A integrates one N-channel MOSFET and one

P-channel MOSFET to realize a synchronous boost converter.

Because the commonly used discrete Schottky rectifier is

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 Diagram” on page 2) is used to reverse the

polarity of the P-channel body diode when the device is shut

down. Thus, 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.

Soft-Start

The soft start-up duration is the time between the device being

device is enabled, the start-up cycle starts with a linear phase.

During the linear phase, the rectifying switch is turned ON in a

current limited configuration, delivering about 350mA, until the

output capacitor is charged to approximately 90% of the input

voltage. At this point, PWM operation begins in Boost mode. If

the output voltage is below 2.3V, PWM switching is done at a

fixed duty-cycle of 75% until the output voltage reaches 2.3V.

When the output voltage exceeds 2.3V, the closed-loop current

mode PWM loop overrides the duty cycle until the output voltage

is regulated. Peak inductor current is ramped to the final value

(typically 1.3A) during the soft-start period to limit inrush current

from the input source. Fault monitoring begins approximately

2ms after the device is enabled.

Over-Temperature Protection (OTP)

The device offers over-temperature protection. A temperature

sensor circuit is integrated and monitors the internal IC

temperature. Once the temperature exceeds the preset threshold

(typically +150°C), the IC shuts down immediately. The OTP has

a typical hysteresis of +25°C. When the device temperature

decreases by this, the device starts operating.

Printed Circuit Board Layout

Recommendations

The ISL9113A is a high-frequency, switching boost converter.

Accordingly, the converter has fast voltage change and high

switching current that may cause EMI and stability issues if the

layout is not done properly. Therefore, careful layout is critical to

minimize the trace inductance and reduce the area of the power

loop.

Power components, such as input capacitor, inductor, and output

capacitor, should be placed close to the device. Board traces that

carry high switching current should be routed wide and short. A

solid power ground plane is important for EMI suppression.

The switching node (SW pin) of the converter and the traces

connected to this pin are very noisy. Noise sensitive traces, such

as the FB trace, should be kept away from SW node. The voltage

divider should be placed close to the FB pin to prevent noise

pickup. Figure 4 on page 6 shows the recommended PCB layout.

In the 8 Ld DFN package, the heat generated in the device is

mainly dissipated through the thermal pad. Maximizing the