LM2678 — 5A 260kHz Step-Down (Buck) Switching Regulator

Overview

The LM2678 is a $5.0\text{ A}$ step-down (buck) switching regulator from Texas Instruments' SIMPLE SWITCHER® family. Capable of driving loads up to $5.0\text{ A}$ with efficiency up to $92\%$ , it features an integrated $0.2\ \Omega$ DMOS power switch operating at a fixed switching frequency of $260\text{ kHz}$ .

The LM2678 accepts input voltages from $8.0\text{ V}$ to $40.0\text{ V}$ and delivers adjustable output voltages down to $1.21\text{ V}$ (or fixed 3.3V, 5.0V, 12V outputs). Its higher switching frequency ( $260\text{ kHz}$ ) compared to classic 52kHz regulators enables smaller filter inductors and capacitors while handling high power.

Quick reference


Regulator Type	Monolithic High-Current Step-Down (Buck) Regulator
Package	TO-263-7 (D2PAK-7 Surface-Mount) / TO-220-7 / VSON-14
Input Voltage Range ( $V_{IN}$ )	$8.0\text{ V}$ to $40.0\text{ V}$ DC
Output Voltage Range ( $V_{OUT}$ )	$1.21\text{ V}$ to $37.0\text{ V}$ DC (Adjustable) / Fixed 3.3V, 5V, 12V
Continuous Output Current ( $I_{OUT}$ )	Up to $5.0\text{ A}$
Switching Frequency	$260\text{ kHz}$ internal fixed oscillator
Conversion Efficiency	$85\% \dots 92\%$
Control Features	Logic `ON`/`OFF` shutdown control, thermal shutdown, current limiting

Pinout (TO-263-7 / TO-220-7 Package)

text

        ┌─────────────────────┐
        │     LM2678S-ADJ     │  (Front Package Face)
        └─┬──┬──┬──┬──┬──┬────┘
          1  2  3  4  5  6  7
         SW VIN CB GND NC FB ON/OFF

Pin	Name	Description
1	`SWITCH OUT` / `SW`	Internal DMOS power switch output connecting to Schottky diode & inductor
2	`VIN`	Unregulated DC power supply input (+8.0V to +40V DC)
3	`CB` / `BOOST`	Bootstrap capacitor connection to `SW` ( $0.01\ \mu\text{F}$ / 10nF ceramic cap)
4	`GND`	Ground reference (connected to tab/exposed pad)
5	`NC`	No internal connection
6	`FEEDBACK` / `FB`	Feedback sensing input ( $1.21\text{V}$ reference voltage for ADJ version)
7	`ON`/`OFF`	Shutdown control input pin (Low = ON, High = OFF)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Input Voltage Range	$V_{IN}$	8.0	12 / 24	40	V	Operating DC input range
Feedback Reference Voltage	$V_{FB}$	1.174	1.210	1.246	V	$V_{IN} = 12\text{V}, I_{OUT} = 2.5\text{A}$
Continuous Output Current	$I_{OUT}$	—	5.0	—	A	$V_{IN} - V_{OUT} \ge 3.0\text{V}$
Peak Current Limit	$I_{CL}$	6.5	8.0	10.0	A	Internal switch peak current limit
DMOS Switch On-Resistance	$R_{DS(ON)}$	—	0.20	0.35	$\Omega$	$I_{OUT} = 5.0\text{A}$
Switching Frequency	$f_{SW}$	225	260	290	kHz	Internal clock frequency
Standby Current	$I_{STBY}$	—	50	150	$\mu\text{A}$	Shutdown state ( $V_{ON/OFF} = 3.0\text{V}$ )

Typical Application Circuit

text

       +V_IN (8.0V - 40V DC Input)
          │
       [Pin 2: VIN]
        LM2678S-ADJ ── [Pin 1: SW] ─── [ 22µH - 47µH Inductor ] ──┬─── +V_OUT Regulated Output
          │     │                            │                     │
          │  [CB: 10nF]                      │                  [ R1 = 1kΩ ]
          │     │                            │                     │
       [Pin 4: GND]               [1N5825 / B540 Schottky]        │
          │                                  │                     │
         GND ────────────────────────────────┴─────────────────────┼─── [Pin 6: FB]
                                                                   │
                                                                 [ R2 ]
                                                                   │
                                                                  GND

$V_{OUT} = 1.21\text{V} \times \left( 1 + \frac{R_2}{R_1} \right)$

Common mistakes

Omitting the Bootstrap capacitor (CB): A $10\text{ nF}$ ceramic capacitor must be connected between Pin 3 (CB) and Pin 1 (SW) to supply gate voltage for the internal DMOS switch. Without it, the switch will not turn on properly.
Inadequate PCB copper area for heatsinking: Delivering 5A output creates thermal dissipation even at 90% efficiency. Solder the TO-263 tab to a large copper ground plane with thermal vias to prevent over-temperature shutdown.

Notes

LM2678 vs LM2596: The LM2678 handles 5A continuous (vs 3A for LM2596) and operates at $260\text{ kHz}$ with higher efficiency due to its low-resistance DMOS switch.

Interface	7-Terminal Switching Output
Package	TO-263-7 (D2PAK-7) / TO-220-7 / VSON-14