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JAJSUV0 June 2024 LM5171

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6.3.5.3 Channel Current Setting Commands (ISET1 and ISET2)

Each channel of the LM5171 has the independent current setting pin, ISET1 and ISET2, for independent channel operation. For multiphase in parallel operation, connect all ISET pins together to achieve balanced power sharing among multiphases.

Each ISET pin accepts an analog voltage to program the channel dc current, as shown in Figure 6-3. The ISET pin signal feeds directly to one of the two non-inverting inputs of the gm amplifier. The other non-inverting pin is controlled by the soft-start. The current sense signal internally has an 1V dc offset voltage before it feeds to the inverting input of the gm amplifier. The channel DC current that flows through the current sense resistor is determined by Equation 1 in the closed loop operation.

Equation 1.

V_{C S_d c} = 0.025 \times (V_{I S E T} - 1 V)

Or by Equation 2,

Equation 2.

I_c h a n n e l_d c = \frac{V_{C S_d c}}{R c s}

Or by Equation 3,

Equation 3.

I_c h a n n e l_d c = \frac{0.025 \times {(V}_{I S E T} - 1 V)}{R c s}

Where

Rcs is the channel current sensing resistor value.

If a PWM signal is provided to control the channel current but no DAC is available, it is recommend to use a two-stage RC filter to convert it to the analog voltage at the ISET, as shown in Figure 6-4. The corner frequency of the filter must be set to at least 1 decade below the PWM frequency in order to attenuate the ripple voltage to less than 1% at the ISET pin, namely, the RC selection satisfies Equation 4, and the PWM to analog voltage conversion is given by Equation 5.

LM5171 Analog Voltage Channel Current Programming

Figure 6-3 Analog Voltage Channel Current Programming

Figure 6-4 PWM Channel Current Programming

Equation 4.

6.28 \times R_{I S E T A} \times C_{I S E T A} \leq 10 \times F_{P W M}

Equation 5.

V_{I S E T} = \frac{R_{P D}}{R_{P D} + 2 \times R_{I S E T}} \times V_{P W M H} \times D_{P W M}

Where

F_PWM is the PWM command frequency.
R_PD is the internal pulldown resistor, which is 1000kΩ typical.
V_PWMH is the PWM signal magnitude.
D_PWM is the PWM signal duty cycle.

Note that the internal pulldown resistor R_PD has some tolerances. If R_ISET << R_PD, the effects of the tolerance on V_ISET accuracy can be greatly reduced.

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6.3.5.3 Channel Current Setting Commands (ISET1 and ISET2)