ADPL74101 datasheet(24/49 Pages) AD

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Data Sheet

ADPL74101

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Current Sense Selection

The ADPL74101 can be configured to use either inductor DC resistance (DCR) sensing or low-value resistor sensing.

The choice between the two current-sensing schemes is a design trade-off between cost, power consumption, and

accuracy. DCR sensing is popular because it saves expensive current-sensing resistors and is more power-efficient,

particularly in high-current applications. However, current sensing resistors provide the most accurate current limits

for the controller. The selection of other external components is driven by the load requirement and begins with the

selection of RSENSE (if RSENSE is used) and the inductor value.

The SENSE+ and SENSE− pins are the inputs to the current comparator. The common-mode voltage range on these

pins is 0V to 65V (See the Absolute Maximum Ratings for more details), enabling the ADPL74101 to regulate output

voltages up to a maximum of 60V. The SENSE+ pin is high impedance, drawing less than ≈1μA. This high impedance

allows the current comparator to be used in inductor DCR sensing. The impedance of the SENSE− pin changes

depending on the common-mode voltage. When less than INTVCC – 0.5 V, the SENSE− pin is at a relatively high

impedance, drawing ≈1μA. When the SENSE− pin is above INTVCC + 0.5 V, a higher current (≈700μA) flows into the pin.

Between INTVCC – 0.5 V and INTVCC + 0.5 V, the current transitions from the smaller current to the higher current. The

SENSE− pin has an additional ≈70μA current when its voltage is above 3.2V to bias internal circuitry from VOUT instead

of VIN, which reduces the input-referred supply current.

Filter components mutual to the sense lines must be placed close to the ADPL74101, and the sense lines must run

close together to a Kelvin connection underneath the current sense element, as shown in Figure 29. Sensing current

elsewhere can effectively add parasitic inductance and capacitance to the current-sense element, degrading the

information at the sense terminals and making the programmed current limit unpredictable. If DCR sensing is used,

as shown in Figure 31. The R1 resistor must be placed close to the switching node to prevent noise from coupling into

sensitive small signal nodes.

Figure 29. Sense Lines Placement with Inductor or Sense Resistor

Low Value Resistor Current Sensing

Figure 30 shows a typical sensing circuit using a discrete resistor. RSENSE is chosen based on the required output

current. The current comparator of the controller has a VSENSE(MAX)of 50mV, 25mV, or 75mV, depending on the state of

the ILIM pin. The current comparator threshold voltage sets the peak inductor current.

Using the maximum inductor current (IL(MAX)) and ripple current (ΔIL) (as described in the Inductor Value Calculation

section), the target sense resistor value is given by Equation 2, as follows:

RSENSE =

VSENSE(MAX)

IL(MAX)+

∆IL

(2)

To ensure that the application delivers the full load current over the full operating temperature range, choose the

minimum value for VSENSE(MAX) as shown in Table 1 (Electrical Characteristics table).

TO SENSE FILTER

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

CURRENT FLOW

ADPL74101 Datasheet(PDF) 24 Page - Analog Devices

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