SL40307A-R19KHF datasheet(21/53 Pages) IRF

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IR3846

21 www.irf.com

August 01, 2013

the sampled current is higher than the OC Limit, an

OC event is detected.

When an Over Current event is detected, the

converter enters hiccup mode.

Hiccup mode is

performed by latching the OC signal and pulling the

Intl_SS signal to ground for 20.48 mS (typ.). OC

signal clears after the completion of hiccup mode and

the converter attempts to return to the nominal output

voltage using a soft start sequence. The converter will

repeat hiccup mode and attempt to recover until the

overload or short circuit condition is removed.

Because the IR3846 uses valley current sensing, the

actual DC output current limit will be greater than OC

limit. The DC output current is approximately half of

peak to peak inductor ripple current above selected

OC limit. OC Limit, inductor value, input voltage,

output voltage and switching frequency are used to

calculate the DC output current limit for the converter.

Equation (2) to determine the approximate DC output

current limit.

LIMIT

OCP







(2)

IOCP

= DC current limit hiccup point

ILIMIT

= Current Limit Valley Point

∆i

= Inductor ripple current

Figure 11: Timing Diagram for Current Limit Hiccup

THERMAL SHUTDOWN

Temperature sensing is provided inside IR3846. The

trip threshold is typically 145

oC. When trip threshold is

exceeded, thermal shutdown turns off both MOSFETs

and resets the internal soft start.

Automatic restart is initiated when the sensed

temperature drops within the operating range. There

is a 20

oC hysteresis in the thermal shutdown

threshold.

REMOTE VOLTAGE SENSING

True differential remote sensing in the feedback loop

is critical to high current applications where the output

voltage across the load may differ from the output

voltage measured locally across an output capacitor

at the output inductor, and to applications that require

die voltage sensing.

The RS+ and RS- pins of the IR3846 form the inputs

to a remote sense differential amplifier (RSA) with

high speed, low input offset and low input bias current

which ensure accurate voltage sensing and fast

transient response in such applications.

The input range for the differential amplifier is limited

to 1.5V below the VCC rail.

Note that IR3846

incorporates a smart LDO which switches the VCC rail

voltage depending on the loading. When determining

the input range assume the part is in light load and

using the lower VCC rail voltage.

There are two remote sense configurations that are

usually implemented.

Figure 12 shows a general

remote sense (RS) configuration. This configuration

allows the RSA to monitor output voltages above

VCC. A resistor divider is placed in between the

output and the RSA to provide a lower input voltage to

the RSA inputs.

Typically, the resistor divider is

calculated to provide VREF (0.6V) across the RSA

inputs which is then outputted to RSo.

The input

impedance of the RSA is 63 KOhms typically and

should be accounted for when determining values for

the resistor divider. To account for the input

impedance, assume a 63 KOhm resistor in parallel to

the lower resistor in the divider network.

The

compensation is then designed for 0.6V to match the

RSo value.

Low voltage applications can use the second remote

sense configuration. When the output voltage range

is within the RSA input specifications, no resistor

divider is needed in between the converter output and

RSA. The second configuration is shown in Figure

13. The RSA is used as a unity gain buffer and

compensation is determined normally.

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