LM5161PWPR

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1. Description

The LM5161PWPR is a 100-V, 1-A synchronous step-down converter with integrated high-side and low-side MOSFETs. The constant-ON-time control scheme requires no loop compensation and supports high step-down ratios with fast transient response. An internal feedback amplifier maintains ±1% output voltage regulation over the entire operating temperature range. The ON-time varies inversely with input voltage resulting in nearly constant switching frequency. Peak and valley current limit circuits protect against overload conditions. The undervoltage lockout (EN/UVLO) circuit provides independently adjustable input undervoltage threshold and hysteresis. The FPWM input pin in LM5161 selects either the forced continuous conduction mode (CCM) under all load levels or the discontinuous conduction mode (DCM) under light or no load conditions. When operating in forced CCM, the LM5161 supports the multiple output and isolated Fly-Buck applications. When programmed for the DCM operation, the LM5161 provides a tightly regulated buck output without any additional external feedback ripple injection circuit.

2. Features

    1. Wide 4.5-V to 100-V Input Voltage Range

    2. Integrated High-Side and Low-Side Switches

        – No Schottky Diode Required

    3. 1-A Maximum Load Current

    4. Constant ON-Time Control

        – No External Loop Compensation

        – Fast Transient Response

    5. Selectable DCM Buck Operation at Light Load

    6. CCM Option Supports Multi-Output Fly-Buck™

    7. No External Ripple circuit needed (at FPWM = 0)

    8. Nearly Constant Switching Frequency

    9. Frequency Adjustable Up to 1 MHz

  10. Programmable Soft-Start Time

  11. Prebias Start-Up

  12. Peak Current Limiting Protection

  13. Adjustable Input UVLO and Hysteresis

  14. ±1% Feedback Voltage Reference

  15. Thermal Shutdown Protection

  16. Create a Custom Design Using the LM5161 With the WEBENCH® Power Designer

3. Applications

    1. Industrial Programmable Logic Controller

    2. IGBT Gate Drive Bias Supply

    3. Telecom DC-DC Primary and Secondary Side Bias

    4. E-Meter Power Line Communication

    5. Low-Power (<12W), Isolated DC-DC (Fly-Buck)

4. Pin Configuration 

5. Pin Functions

6. Feature Description

1. Control Circuit

The LM5161 step-down switching regulator employs a control principle based on a comparator and a one-shot ON-timer, with the output voltage feedback (FB) compared to the voltage at the Soft-Start (SS) pin (VSS). If the FB voltage is below VSS, the internal buck switch is turned on for a time period determined by the input voltage and one-shot programming resistor (RON). Following the ON-time, the buck switch must remain off for the minimum OFF-time forced by the minimum OFF-time one-shot. The buck switch remains off until the FB voltage falls below VSS again, when it turns on for another ON-time one-shot period. During a rapid start-up or when the load current increases suddenly, the regulator operates with minimum offtime per cycle. When regulating the output in steady state operation, the off-time automatically adjusts to produce the SW pin duty cycle required for output voltage regulation.

2. VCC Regulator

The LM5161 contains an internal high voltage linear regulator with a nominal output voltage of 7.3 V (typical). The VCC regulator is internally current limited to 30 mA (minimum). This regulator supplies power to internal circuit blocks including the synchronous FET gate driver and the logic circuits. When the voltage on the VCC pin reaches the undervoltage lockout (VCC(UV)) threshold of 3.98 V (typical), the IC is enabled. An external capacitor at the VCC pin stabilizes the regulator and supplies transient VCC current to the gate drivers. An internal diode connected from VCC to the BST pin replenishes the charge in the high-side gate drive bootstrap capacitor when the SW pin is low.

3. Regulation Comparator

The feedback voltage at the FB pin is compared to the SS pin voltage VSS. In normal operation when the output voltage is in regulation, an ON-time period is initiated when the voltage at FB pin falls below VSS. The high-side buck switch stays on for the ON-time one-shot period causing the FB voltage to rise. After the on-time period expires, the high-side switch will remain off until the FB voltage falls below VSS. During start-up, the FB voltage is below VSS at the end of each on-time period and the high-side switch turns on again after the minimum forced off-time of 170 ns (typical). When the output is shorted to ground (FB = 0 V), the high side peak current limit is triggered, the high-side FET is turned off, and remains off for a period determined by the current limit OFF-time one-shot. See the Current Limit section for additional information.

4. Soft-Start

The soft-start feature of the LM5161 allows the converter to gradually reach a steady-state operating point, thereby reducing start-up stresses and current surges. When the EN/UVLO pin is above the EN/UVLO standby threshold VUVLO(TH) = 1.24 V (typ) and VCC exceeds the VCC undervoltage VCC(UV) = 3.98 V (typ) threshold, an internal 10-µA current source charges the ex ternal capacitor at the SS pin (CSS) from 0 V to 2 V. The voltage at the SS pin is connected to the noninverting input of the internal FB comparator. The soft-start interval ends when the SS capacitor is charged to the 2 V reference level. The ramping voltage at the SS pin produces a controlled, monotonic output voltage start-up. A minimum 1-nF soft-start capacitor must be used in all applications.

5. Error Transconductance (GM) Amplifier

The LM5161 provides a trans-conductance (GM) error amplifier that minimizes the difference between the reference voltage (VREF) and the average feedback (FB) voltage. This amplifier reduces the load and line regulation errors that are common in constant-on-time regulators. The soft-start capacitor (CSS) provides compensation for this error correction loop. The soft-start capacitor should be greater than 1 nF to ensure stability.

6. N-Channel Buck Switch and Driver

The LM5161 integrates an N-channel buck switch and associated floating high-side gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage bootstrap diode. A 10-nF or larger ceramic capacitor connected between the BST pin and the SW pin provides the voltage to the high-side driver during the buck switch ON-time. During the OFF-time, the SW node is pulled down to approximately 0 V and the bootstrap capacitor charges from VCC through the internal bootstrap diode. The minimum OFF-time of 170 ns (typical) provides a minimum time each cycle to recharge the bootstrap capacitor.