Optimizing Line and Load Regulation in Automotive, 48-V Battery and eBike Applications

Abstract

Higher voltage batteries are becoming more prevalent in automotive and eBike applications. The extended V_IN range is causing designers to examine more closely the line regulation of the pre-regulator. Furthermore, the pre-regulator might need to support a no load to full load condition, so load regulation is also of high importance. This application note reviews line and load regulation for 48-V battery and eBike, pre-regulators using LM5013-Q1 as an example.

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1 Introduction

A common control topology employed in buck regulators is constant-on-time (COT). This control topology is being adopted in several 48-V battery and eBike, pre-regulators such as LM5013-Q1. This application note is an overview of how line and load regulation can be optimized with LM5013-Q1 for wide-V_IN range applications, specifically, the 48 V battery and eBike applications.

2 Overview of COT

Figure 2-1 produces a regulated output voltage by using its feedback comparator to servo the output by comparing the sampled output voltage, V_FB, and the reference voltage, V_REF. The feedback comparator issues an on-time pulse, after it trips. This cycle continues at the programmed switching frequency for the set output voltage, dictated by the on-time (R_RON) and feedback (R_FBT, R_FBB) resistors. Equation 1 and Equation 2 illustrate the necessary calculations.

Figure 2-1 LM5013 Functional Block Diagram

Equation 1.

$R_{R O N} (k Ω) = \frac{V_{O U T} (V) \times 2500}{F_{S W} (k H z)}$

Equation 2.

$R_{F B B} = \frac{V_{R E F}}{V_{O U T} - V_{R E F}} \times R_{F B T}$

The feedback comparators terminal connected to the FB node must have sufficient ripple such that during the on-time, the FB node can be charged sufficiently above the reference voltage (V_REF). This outcome is achieved with example ripple injection circuit Figure 2-2.

Figure 2-2 LM5013 Design With External Ripple Injection

The integrator (R_A, C_A) generates a voltage ramp , in-phase with the inductor current and is AC coupled with C_B. By following Equation 3, Equation 4, and Equation 5 sufficient ramp amplitude is applied to the feedback node for stability and transient performance is optimized. Note, the recommended minimum ramp amplitude, 12 mV in the case of LM5013, is advised for the device's given hysteresis, along with margin for reduced sensitivity to noise. Please refer to the LM5013-Q1 Automotive 100-V Input, 3.5-A Non-Synchronous Buck DC/DC Converter with Ultra-low IQ data sheet for the recommended value and further clarification of terms. Additional theory on this design is provide in the Stability Analysis and Design of COT Type-3 Ripple Circuit application note.

Equation 3.

$C_{A} \geq \frac{10}{F_{S W} \times (R_{F B T} | | R_{F B B})}$

Equation 4.

$V_{R A M P} = \frac{(V_{I N} - V_{O U T}) \times t_{O N}}{R_{A} C_{A}}$

Equation 5.

$C_{B} \geq \frac{t_{T R}}{3 \times R_{F B T}}$

3 Line Regulation

The injected ramp waveform leads to the average feedback voltage exceeding the reference voltage for which the output voltage was programmed with. Additionally, the waveform’s peak (Equation 4) is directly proportional to the input voltage, leading to further variance as V_IN changes. This variance is gained up through the feedback divider, resulting in the output voltage varying from the set point. Figure 3-1 and Table 3-1 gives a conceptual and quantitative measure how higher, peak ramp amplitude results in an increase in average feedback voltage (V_FB(AVG)).

Figure 3-1 Peak Ramp Amplitude vs V_FB(AVG)

Table 3-1 Measured Peak Ramp Amplitude v. V_FB(AVG)

Ramp Amplitude, V_PEAK (mV)	V_FB(AVG) (V)
13.488	1.204
15.437	1.206
20.342	1.212
23.353	1.217
23.945	1.216