LM2648 datasheet(16/23 Pages) NSC

16 / 23 page

Operation Descriptions (Continued)

ACTIVE OUTPUT DISCHARGE

Each channel has an embedded MOSFET with the drain

connected to the SWx pin to provide a smooth controlled

shutdown ramp. Each MOSFET will discharge the output

capacitor of its channel if the device enters a fault state

caused by one of the following conditions:

UVP or Prot-in2

UVLO (Internal or External)

Thermal shut-down (TSD)

Prot-in1

The MOSFETS provide 500

Ω of discharge resistivity for

channels 1 and 2 and 200

Ω for channel 3.

Channel 3 has a secondary quick discharge feature. When

the channel 3 output voltage falls below 65% of nominal,

LDRV3a is turned on (LDRV3b remains off). This provides a

faster discharge for the second half of the channel 3 shut-

down process.

CURRENT SENSING

As shown in Figure 2, the KSx and RSNSx pins are the

inputs of the current sense amplifier for channel 3. Current

sensing is accomplished by sensing the voltage across a

current sense resistor connected from V

IN to the drain of the

top FET. Keeping the differential current-sense voltage be-

low 200mV ensures linear operation of the current sense

amplifier. Therefore, the current sense resistor must be small

enough so that the current sense voltage does not exceed

200mV when the top FET is on. There is a leading edge

blanking circuit that forces the top FET on for at least 166ns.

Beyond this minimum on time, the output of the PWM com-

parator is used to turn off the top FET. Additionally, a mini-

mum voltage of at least 50mV across the sense resistor is

recommended to ensure a high SNR at the current sense

amplifier.

Assuming a maximum sense voltage of 200mV, the current

sense resistor can be calculated as follows:

where Imax is the maximum expected load current, including

overload multiplier (ie:120%), and Irip is the inductor ripple

current (See Inductor Selection below). The above equation

gives the maximum recommended value for the sense resis-

tor, Rsns. Conduction losses will increase with Rsns, thus

lowering efficiency.

To ensure accurate current sensing, special attention in

board layout is required. The KSx and RSNSx pins require

separate traces to form a Kelvin connection to the corre-

sponding current sense nodes.

Current sensing on channels 1 and 2 is used only to monitor

the current limit. Current is sensed identical to channel 3,

except that it does not require a KS pin and there is no

blanking time. As with channel 3, RSNSx is connected to the

bottom side of the current sense resistor. The top side of the

current sense resistor connects to both VIN and the current

limit resistor. Both KSx and RSNSx must be connected with

separate traces to ensure accurate current limiting.

CURRENT LIMITING

The peak current limit is set with an external resistor con-

nected between the ILIMx pin and the top of the current

sense resistor. An adjustable internal current sink on the

ILIMx pin produces a voltage across the ILIM resistor to set

the current limit threshold. This voltage is compared with the

current sense voltage. A 10nF capacitor across the resistor

is required to filter unwanted noise that could improperly trip

the current limit comparator.

Current limit is activated when the inductor current is high

enough to cause the voltage across the current sense resis-

tor to exceed the voltage across the current limit resistor.

This will toggle the comparator, which turns off the top FET

immediately. The current limit comparator is disabled when

the top FET is turned off and during the leading edge blank-

ing time. The equation for current limit resistor, Rlim, is as

follows:

Where I

max is the load current at which the current limit

comparator will be tripped, and I

REF is the reference sink

current (20µA typical). This calculated Rlim value guarantees

that the minimum current limit will not be less than I

max.Itis

recommended that a 1% tolerance resistor be used.

REFERENCE CURRENT SETTING

The ILIMx current sink value can be adjusted using the

RILIM pin. The RILIM pin is connected through a resistor to

SGND to set the reference current at the current limit pins

(ILIMx). The resistor value can be determined using the

following equation:

Where I

REF is the desired reference current. Generally, a

default value of 61.9k

Ω is used, which provides the recom-

mended reference current of 20µA. Once an initial value is

selected, R

IREF can be adjusted to precisely trim the current

limit setting on all three channels.

20059610

FIGURE 2. Ch.3 Current Sense and Current Limit

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