ISL8013A datasheet(13/17 Pages) INTERSIL

13 / 17 page

FN7526.0

November 25, 2009

block, only affects the operation during the start-up and

will be discussed separately. The error amplifier is a

transconductance amplifier that converts the voltage

error signal to a current output. The voltage loop is

internally compensated with the 27pF and 390kΩ RC

network. The maximum EAMP voltage output is precisely

clamped to 1.6V.

SKIP Mode

Pulling the SYNCH pin LO (<0.4V) forces the converter

into PFM mode. The ISL8013A enters a pulse-skipping

mode at light load to minimize the switching loss by

reducing the switching frequency. Figure 37 illustrates

the skip-mode operation. A zero-cross sensing circuit

shown in Figure 2 monitors the N-MOSFET current for

zero crossing. When 8 consecutive cycles of the inductor

current crossing zero are detected, the regulator enters

the skip mode. During the eight detecting cycles, the

current in the inductor is allowed to become negative.

The counter is reset to zero when the current in any cycle

does not cross zero.

Once the skip mode is entered, the pulse modulation

starts being controlled by the SKIP comparator shown in

Figure 2. Each pulse cycle is still synchronized by the

PWM clock. The P-MOSFET is turned on at the clock's

rising edge and turned off when the output is higher than

1.5% of the nominal regulation or when its current

reaches the peak Skip current limit value. Then the

inductor current is discharging to 0A and stays at zero.

The internal clock is disabled.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

rising edge of the internal clock as it repeats 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 SYNCH pin.

The falling edge on the SYNCH triggers the rising edge of

the LX pulse. Make sure that the minimum on time of the

LX node is greater than 140ns.

Overcurrent Protection

The overcurrent protection is realized by monitoring the

CSA output with the OCP comparator, as shown in

Figure 2. The current sensing circuit has a gain of

250mV/A, from the P-MOSFET current to the CSA

output. When the CSA output reaches 1.4V, which is

equivalent to 4.8A for the switch current, 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

MOSFET.

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. Upon

detection of the initial overcurrent condition, the

overcurrent fault counter is set to 1. If, on the

subsequent cycle, another overcurrent condition is

detected, the OC fault counter will be incremented. If

there are 17 sequential OC fault detections, the regulator

will be shut down under an overcurrent fault condition.

An overcurrent fault condition will result in the regulator

attempting to restart in a hiccup mode within the delay of

four soft-start periods. At the end of the fourth soft-start

wait period, the fault counters are reset and soft-start is

attempted again. If the overcurrent condition goes away

during the delay of four soft-start periods, the output will

resume back into regulation point after hiccup mode

expires.

Short-Circuit Protection

The 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.

FIGURE 36. PWM OPERATION WAVEFORMS

VEAMP

VCSA

DUTY

CYCLE

VOUT

ISL8013A

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