Buck converter (step-down regulator IC)

01

Match input range, output and current

TI's TPS54360 datasheet describes a 60 V, 3.5 A step-down regulator with an integrated high-side MOSFET (92 mΩ), current-mode control, an input range of 4.5–60 V (65 V absolute maximum), survival of load-dump pulses up to 65 V per ISO 7637, 3.5 A continuous output with a 4.5 A minimum peak inductor current limit, a 0.8 V ±1% reference, adjustable UVLO and hysteresis, internal soft start, cycle-by-cycle current limit, and thermal, overvoltage and frequency-foldback protection, in an 8-pin HSOIC PowerPAD package rated −40 to 150 °C junction, for 12, 24 and 48 V industrial, automotive and communications systems. [1]

Linked suppliers list a 4–60 V input, 600 mA non-synchronous buck converter with internal soft start, loop compensation and output over-voltage protection [2], and step-down DC/DC converter ranges among their power ICs. [3]

Editorial note: size the input range to the worst transient on your bus, not only its nominal value, and state the peak output current for start-up and load steps as well as the continuous current. [1]

Give the supplier the input range and transients, output voltage and tolerance, continuous and peak current, switching frequency or range, efficiency targets at light and full load, ripple limit, synchronisation, enable/UVLO and power-good needs, protection, package and thermal limits, temperature range, qualification (industrial, AEC-Q100) and quantity.

02

Choose switching frequency and the inductor and capacitors

The TPS54360 switches at a fixed 100 kHz to 2.5 MHz, can synchronise to an external clock, has a 135 ns minimum controllable on-time at 12 V and 25 °C, draws 146 µA operating and 2 µA shutdown current, and uses pulse-skipping Eco-mode for efficiency at light loads, with an integrated boot-recharge FET for low dropout at light load. [1]

Its design procedure sizes the minimum output inductor as L ≥ (VIN(max) − VOUT) ÷ (IOUT × KIND) × VOUT ÷ (VIN(max) × fSW), where KIND is the inductor ripple current as a fraction of maximum output current — up to 0.3 with low-ESR ceramic output capacitors and 0.2 with higher-ESR capacitors — keeping ripple above 150 mA for stable current-mode operation; for 60 V in, 5 V out, 3.5 A and 600 kHz it reaches 7.3 µH and selects 8.2 µH, with 3.5 A RMS and 3.97 A peak, and advises an inductor saturation rating at or above the 5.5 A nominal switch current limit for a conservative design. [1]

Editorial calculation: (60 − 5) ÷ (3.5 × 0.3) × 5 ÷ (60 × 600,000) ≈ 7.3 µH, matching the datasheet. At 600 kHz a 135 ns minimum on-time is a minimum duty cycle of about 8%, so from 60 V the lowest regulated output at that frequency is about 4.9 V; a lower output from 60 V needs a lower switching frequency. [1]

03

Break the efficiency figure into its losses

Richtek Technology's application note AN005 divides the losses of a synchronous buck converter in continuous conduction mode into four groups: power semiconductor loss, inductor loss, gate driver loss and PCB trace loss. For the high-side MOSFET it counts switching-on, switching-off and conduction loss, the last set by on-resistance and RMS current; for the low-side MOSFET, conduction, dead-time body-diode and reverse-recovery loss. Inductor loss is DC (winding resistance) plus core loss, which depends on operating frequency and total flux swing; gate driver loss depends on total gate charge, driver voltage and switching frequency. [4]

The note checks its equations against a bench converter: an RT8120 single-phase synchronous buck PWM controller converting 12 V to 1.2 V at 300 kHz with a 12 V driver supply, a 1 µH inductor of 1.2 mΩ DCR, one BSC090N03LS high-side MOSFET and two BSC090N03LS low-side MOSFETs in parallel, and plots measured against calculated efficiency and the loss of each component versus output current. [4]

Editorial calculation for that design: duty cycle is 1.2 ÷ 12 = 10%, so the high-side on-time is about 333 ns of each 3.33 µs period; inductor ripple is (12 − 1.2) × 0.10 ÷ (1 µH × 300 kHz) = 3.6 A peak to peak. At an assumed 20 A load the inductor DC loss alone is 20² × 0.0012 = 0.48 W, 2% of the 24 W output. [4]

Editorial note: ask the supplier for efficiency curves measured at your input, output and frequency, and for the inductor and MOSFETs used on the evaluation board, since Richtek's breakdown shows these external parts carry a share of the loss.

04

Compare what suppliers offer

Suppliers linked to this page list buck converters; each card links to the page reviewed.

The listings reviewed cover wide-input non-synchronous buck converters and step-down DC/DC converter families. Ask each for the datasheet with efficiency curves, minimum on-time, current limit and thermal data at your conditions. [2] [3]

05

Trace what each part controls

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your rail

A buck converter quotation needs values for your rail. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the bus, rail and current data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Input: [range; transients]
Output: [V; tolerance; ripple]
Current: [continuous; peak]
Frequency: [kHz; sync]
Efficiency: [light; full load]
Protection: [limits; OVP]
Package/grade: [package; AEC-Q100]
Quantity: [units]

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