アプリケーションノート 7068

# How to Calculate Sense Resistor Values for Custom Load Current Requirements in Device Power Supply ICs

This application note provides details on calculating new sense resistor values for custom load current requirements within the 200µA to 800mA current range in the MAX9959, which is a device power supply (DPS) IC that provides programmable voltage and current ranges.

#### Introduction

The MAX9959, a device power supply (DPS) IC, with key features such as programmable voltage and current ranges, small size, and independent current and voltage settings. The MAX9959 is a voltage source when the load current is between the two programmed limits, and transitions gracefully into a precision-current source/sink if a programmed current limit is reached.

The MAX9959 programmable-current-range selection feature controls the full-scale current range for either force current (FI) or measure current (MI) mode. This current range selection is controlled by both serial bits in the control word (RS0, RS1, RS2) and the sense resistor values.

There is always a possibility that a customer needs a custom current range between 200µA to 800mA for a specific application. This application note describes how to calculate new sense resistor values for a custom load current requirement in DPS ICs.

#### Calculating the Sense Resistor Value for Custom Load Current Requirements

Table 1 provides four current ranges, their corresponding serial bits, and the nominal sense resistor values.

Table 1. Programmable Current Range Selection Bits and Nominal Sense Resistor Values

Data Bits Range Maximum Current Nominal Sense Registor Value
RS2
(D13)
RS1
(D12)
RS0
(D11)
0 0 0 D ±200μA 5000Ω
0 0 1 C ±2mA 500Ω
0 1 1 A ±800mA 1.25Ω
1 X X X - -

In this application note, we consider range D, which has a maximum current of 200µA and a nominal sense resistor value of 5000Ω) to derive our custom sense resistor value formula.

The current sense amplifier (CSA) is a special-purpose amplifier that outputs a voltage proportional to the current flowing in a power rail. Figure 1 shows the CSA in the MAX9959 that monitors the voltage across the sense resistors connected to outputs RA, RB, RC, and RD. For currents within each selected range, the voltage output behaves as a constant voltage source with a maximum load current limit set by using the CSA. View higher resolution image ›
Figure 1. Current sense amplifier in the MAX9959

As shown in Figure 1, the 5000Ω sense resistor at full deflection (200μA) goes through the CSA. The CSA pre-buffer feedback voltage is calculated as follows:

VFB = 200μA × 5000Ω = 1V

The CSA has a gain of 4, so,the output voltage of the CSA is 4 times the MAX9959 input control voltage, as follows:

VIN = Gain × VFB = 4 × 1V = 4V (Equation 1)

In force current mode, the output current is proportional to the input control voltage and behaves according to Equation 2:

IOUT = VIN/(4 × RSENSE)(Equation 2)

From Equation 1, VIN = 4, which is valid in all possible current ranges. Entering VIN into Equation 2, we get Equation 3 to calculate the sense resistor value.

IOUT = 4/(4 × RSENSE)
IOUT = 1/RSENSE
So, RSENSE = 1V/IOUT(Equation 3)

#### Example Calculation

For a 5mA current range example, select range B, which has a maximum current of 20mA, by setting data bits RS2, RS1, and RS0 (0, 1, 0). To support the maximum FSR current of 5mA, use Equation 3 to calculate the required sense resistor value as follows: RSENSE = 1V/5mA (required FSR current) = 200Ω

#### Conclusion

This application note provides brief overview of the current sense amplifier circuit in the MAX9959 DPS IC. System designers can use this methodology or example calculation to design a DPS system having variable load currents.