JAJSVG8 データシート

JAJSVG8 October 2024 LM251772

PRODUCTION DATA

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）

RHA|40
- MPQF135D

サーマルパッド・メカニカル・データ

RHA|40
- QFND660

発注情報

8.3.11.2 Output Current Limitation

In this configuration the current sense has an internal feedback to the peak current limit of the device. The ILIM circuit regulates the ILIMCOMP voltage to V_(ISET), and overrides the voltage loop regulation.

the peak current limit down as long as the differential voltage between ISNSP and ISNSN exceeds the internal offset voltage of the ILIM circuit. The ILIM threshold can be set via the register programming or via the an ISET resistor in the ILIMCOMP pin.

If the current limit threshold is selected by a resistor the regulation overwrites the voltage loop once V_(ISET) is increasing to the threshold voltage (typ. 1V). The threshold voltage for ISET can be calculated with:

Equation 15.

{V_{(ISET)} = (V_{(I S N S P)} - V_{(I S N S N)}) ∙ {g m}_{(ILIMCOMP)} ∙ R}_{(ILIMCOMP)}

Hence the resistor value to select the current limit threshold voltage calculates with:

Equation 16.

R_{(I S E T)} = \frac{V_{(I S E T)}}{(V_{(I S N S P)} {- V_{(I S N S N)}) ∙ g m}_{(I L I M C O M P)}}

For high frequency noise suppression a capacitor based on the following equation should be placed in parallel to R_(ISET)

Equation 17.

C_{(I S E T)} = \frac{1}{π ∙ f_{(S W)} ∙ R_{(I S E T)}}

If the threshold for the current limit is programmed by the internal DAC the bandwidth of the current limit control loop can be optimized for different loads with a resistor and capacitor network on the ILIMCOMP pin. A simple integrator compensation for resistive loads can be selected according the following equations:

Equation 18.

C_{O 2} = \frac{5}{2 ∙ π ∙ f_{b w} ∙ R_{(L O A D)}}

Where C_O2 is the capacitance after the average current sense resistor R_(SNS)

f_bw is the bandwidth of the voltage loop compensation (see Voltage Regulation Loop)

Equation 19.

C_{O 1} = C_{O} - C_{O 2}

Where C_O is the total output capacitance determined by the voltage loop calculation and the applications voltage ripple requirement.

Where C_O1 is the capacitance before the average current sense resistor R_(SNS)

Equation 20.

f_{p} = \frac{1}{2 ∙ π ∙ R_{(S N S)} ∙ C_{O 2}}

Equation 21.

f_{b w i l i m} = {f_{p} ∙ 10}^{- 0.25}

Equation 22.

C_{(I L I M C O M P)} = \frac{{g m}_{(I L I M C O M P)}}{2 π ∙ f_{b w i l i m}}

LM251772 Simplified Schematic current
limit components with resistive load

Figure 8-21 Simplified Schematic current limit components with resistive load

For a electronic load (CC-mode CR-mode) a type II compensation network might be necessary to adopt to the internal regulation loop and bandwidth of the used electronic load. Please refer to the Quick Start Calculator Tool for more detailed optimization.

If the current limit threshold is selected by a resistor instead of the internal DAC the regulation overwrites the voltage loop once V_(ISET) is increasing to the threshold voltage (typ. 1V). The threshold voltage for ISET can be calculated with:

Equation 23.

{V_{(ISET)} = (V_{(I S N S P)} - V_{(I S N S N)}) ∙ {g m}_{(ILIMCOMP)} ∙ R}_{(ISET)}

Hence the resistor value to select the current limit threshold voltage calculates with:

Equation 24.

R_{(I S E T)} = \frac{V_{(I S E T)}}{(V_{(I S N S P)} {- V_{(I S N S N)}) ∙ g m}_{(I L I M C O M P)}}

For high frequency noise suppression a capacitor based on the following equation should be placed in parallel to R_(ISET)

Equation 25.

C_{(I S E T)} = \frac{1}{π ∙ f_{(S W)} ∙ R_{(I S E T)}}

The read-out register value of the ''ILIM_THRESHOLD" control register is clamped for the lower and for the upper limit of the register range.

The reg. readout value is clamped to the lowest clamp current ( e.g. 500mA) if a register value below the value of clamp current been written in before.
The reg. readout value is clamped to the highest clamp current if a register value above the highest value of clamp current has been written in before.