60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

The automotive and industrial markets require power supplies that operate at low heat, fit into tight spaces, and meet low EMI standards. The switching regulators LT8362, LT8364 and LT8361 meet these requirements in boost, SEPIC or inverting topologies. Each device supports a wide input range of 2.8 V to 60 V, suitable for industrial or automotive environments, features low IQ mode (Burst Mode®) capability, and offers optional SSFM to reduce EMI.Built with robust and reliable 60 V/2 A, 60 V/4 A, and 100 V/2 A power switches (operating efficiently at frequencies up to 2 MHz), these devices deliver high power in tight spaces while meeting stringent requirements Thermal and EMI Requirements

By Joey Yurgelon, Jesus Rosales and Mark Marosek

The automotive and industrial markets require power supplies that operate at low heat, fit into tight spaces, and meet low EMI standards. The switching regulators LT8362, LT8364 and LT8361 meet these requirements in boost, SEPIC or inverting topologies. Each device supports a wide input range of 2.8 V to 60 V, suitable for industrial or automotive environments, features low IQ mode (Burst Mode®) capability, and offers optional SSFM to reduce EMI. Built with robust and reliable 60 V/2 A, 60 V/4 A, and 100 V/2 A power switches (operating efficiently at frequencies up to 2 MHz), these devices deliver high power in tight spaces while meeting stringent requirements thermal and EMI requirements.

Automotive Input Transient and Pre-Boost

With the dramatic increase in Electronic content in today’s automobiles, the number of power supplies has multiplied, many of which require direct conversion of a wide range of battery voltages into usable regulated outputs. All devices in the LT836x family have a minimum input voltage of 2.8 V and can operate in cold-crank or start-stop applications; maximum input Voltage capability of
60 V to handle high input voltage transients such as power cuts.

Such a wide input voltage range makes the LT836x family ideal for automotive pre-boost applications. Automotive buck regulators require a pre-boost stage in applications where the battery input voltage may drop below the buck output voltage. The LT8361, LT8362 and LT8364 provide the necessary boost during periods of low battery power, while shutting down at normal battery voltage or during power cuts with extremely low power consumption.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 1. The LT836x converters provide the full slope compensation required for normal operation when the peak switch current limit is reached, while the peak switch current limit does not decrease DC with duty cycle.

Stable and reliable power switch

A key requirement of any switching Regulator is to provide sufficient power for a given application over the entire input voltage range while maintaining reliable operation. These power switches are robust, offering voltage/peak currents of 60 V/2 A (LT8362), 60 V/4 A (LT8364) and 100 V/2 A (LT8361) to support a wide range of applications. In addition to extending the input voltage range of SEPIC and inverting converters, the high power switching voltage ratings of the LT836x family also extend the output voltage capability.

Maximum Power Delivery: Flat Current Limit and Duty Cycle

To maximize power delivery over the entire input voltage range, the LT836x family of power switches maintains the peak switch current limit constant over the entire duty cycle range. What is the advertised switching current, the user can get as much current without any discount. This is a significant advantage over some converters where the peak switch current limit may drop by 30% or more at high duty cycles.

Current-mode DC-DC converters typically slope-compensate the peak switch current limit to avoid subharmonic oscillations when the peak switch current limit is reached. The downside is that as the duty cycle increases (as the input voltage decreases), the peak switch current limit decreases. The LT836x family provides the full slope compensation required for normal operation when the peak switch current limit is reached, while the peak switch current limit does not degrade dc with duty cycle.

2 MHz operating frequency: compact power supplies above the AM band

To meet the need for compact power supplies, DC-DC converters use high switching frequencies to minimize device size and cost. Additionally, automotive applications require operation above the AM band, pushing frequencies up to 2 MHz.

Traditionally, high switching frequencies result in increased switching losses and limited duty cycle range. The LT836x family employs fast power switch drivers to minimize switching losses, and has very short minimum ON and off times to support a wide conversion range even at 2 MHz. For example, many applications traditionally run at 400 kHz for maximum efficiency, and the LT836x family enables lower losses and a higher duty cycle range. The thermal performance of each topology involved (boost, SEPIC, and inverting) is shown in Figure 2.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 2. Thermal performance of the LT8362 Cuk inverting, LT8364 boost, and LT8361 SEPIC designs.

Burst Mode of Operation: High Efficiency at Light Loads

High efficiency at light loads is a key feature in the automotive environment, which is critical for extending battery life. With a selectable burst mode of operation (selectable using the SYNC/MODE pin), the LT836x family offers high efficiency at light loads (see Table 2). At lower switching frequencies, burst mode of operation uses evenly spaced single switching pulses to reduce switching losses while minimizing output voltage ripple. The LT836x family draws as low as 9µA of current from the input pins in deep sleep mode or in pass-through mode in pre-boost applications.

SSFM mode: All three topologies are CISPR 25 Class 5 compliant

The LT836x family is capable of meeting CISPR 25 Class 5 standards using spread spectrum frequency modulation (SSFM) mode and proper board layout with some filtering. 25 Class 5 standard.

In EMI-sensitive environments, designers have traditionally avoided switching regulators. The large capacitance and troublesome hot loops of switching regulators increase the importance of PCB layout to achieve good EMI performance and small size solutions, placing a burden on board design and manufacturing. Factory demo circuits for the LT8362, LT8364, and LT8361 include the necessary input/output filters and an exemplary PCB layout to meet CISPR 25 Class 5 (tested) when SSFM mode is selected (see Table 2). Converters are largely eliminated from EMI considerations, reducing application development time and cost. Figure 4 shows the EMI test results for the boost solution.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 3. A compact low-EMI converter solution.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 4. EMI test results for the LT8364 boost solution.

Best of Both Worlds: Burst Mode and SSFM

Until recently, choosing SSFM mode to reduce EMI also meant that the less efficient pulse-skipping mode had to be used at light loads, but the LT836x family does not need this trade-off. Simply adding a 100kΩ Resistor between the SYNC/MODE pin and ground (see Table 2), the LT836x family seamlessly transitions from SSFM mode to burst operation when the load is light. The result is low EMI and high efficiency at all loads.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 5. Pulse skipping versus burst mode of operation for the LT8362 boost solution (24 V input, 48 V output).

Packages, Pin Compatibility, and Temperature Ratings

For customers who prefer a leaded package, each device is available in a pin-compatible 16(12)-pin MSE TSSOP, with four pins removed to comply with HV pin spacing requirements. For a smaller solution, the LT8362 and LT8364 are also available in DFN packages. The LT8362 (3 mm × 3 mm) 10-pin DFN is pin-compatible with the LT8364 and can be placed on the (4 mm × 3 mm) LT8364 12-pin DFN PCB space (see Figure 6). All packages have a thermally enhanced exposed ground pad and are available in E, I, and H temperature grades.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 6. Pin compatibility of LT8361, LT8362 and LT8364 packages.

Boost/SEPIC/Inverting: FBX pin for positive or negative output

A single FBX pin provides both positive and negative output voltages, enabling all topologies. Inverting applications are just as available as boost or SEPIC, saving design time and effort.

boost converter

Some applications require an output voltage higher than the input voltage, and the LT836x family’s 2.8 V to 60 V input capability and power switch rating range make it ideal for many boost converter applications. For large conversion ratio designs, operation in discontinuous conduction mode (DCM) may be the best solution; continuous conduction mode (CCM) can provide higher output power.

The converter in Figure 7 is the LT8364 low IQ, low EMI, 2 MHz, 24 V boost converter with SSFM and CISPR 25 Class 5 radiated and conducted EMI compliance (Figure 4). With an input voltage of 12 V, the application easily achieves a peak efficiency of 94%.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 7. The LT8364, 2 MHz, 24 V output boost converter is CISPR 25 Class 5 compliant (see Figure 4).

SEPIC converter

The input voltage for automotive and industrial applications may be higher or lower than the desired output voltage. For DC-DC converter applications that need to boost and step down the input, the SEPIC topology is often the solution. SEPIC supports applications that require the output to be disconnected to ensure no output voltage during shutdown, and is tolerant of output short-circuit faults because there is no DC path from input to output. The switches are rated at 60 V/100 V, have short minimum turn-on and turn-off times, and support a wide input voltage range. The LT836x family offers an optional BIAS pin that can be used as a second input supply for the INTVCC regulator to improve efficiency.

The SEPIC converter in Figure 8 uses the LT8361 to demonstrate the versatility of a 100 V rated switch. The switching voltage rating must be greater than the sum of the maximum input and output voltages. With 48 V input and 24 V output, the switch easily handles the required 72 V. When the input is greater than the output, the BIAS pin is connected to VOUT for improved efficiency. When operating in SSFM mode, the application meets CISPR 25 Class 5 radiated and conducted EMI standards (Figure 9). Peak efficiency at 12 V input is 88%.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 8. The LT8361, 400 kHz, 24 V output SEPIC converter is CISPR 25 Class 5 EMI compliant.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 9. EMI test results for the LT8361 SEPIC solution.

Inverting Converter

Negative power supplies are commonly used in today’s electronics. However, for many applications only positive input voltages are available. When configured in an inverting topology, the LT836x family can regulate from positive input voltages above or below the magnitude of the negative output voltage. As with the SEPIC topology, the high switching rating of 60 V/100 V and the short minimum turn-on and turn-off times support a wide input voltage range.

Operating at 2 MHz, the LT8362 provides an easy way to generate a negative voltage from a positive input supply, as shown in Figure 10 – a low IQ, low EMI, 2 MHz, -12 V inverting converter using SSFM. The application can operate from inputs up to 42 V (|VOUT| + VIN60 V) with a robust 60 V switch. At VIN of 12 V, peak efficiency can reach 85%. In SSFM mode of operation, the application meets CISPR 25 Class 5 radiated and conducted EMI standards (Figure 11).

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies

Figure 10. The LT8362, 2 MHz, -12 V output inverting converter is CISPR 25 Class 5 EMI compliant.

60V and 100V, Low IQ Boost/SEPIC/Inverting Converters for Compact, High Efficiency, Low EMI Power Supplies
Figure 11. EMI test results for the LT8362 inverting solution.

in conclusion

To meet the needs of the automotive and industrial markets for compact, high efficiency, low EMI power supplies, the LT836x family offers the robust LT8362 (60 V/2 A), LT8364 (60 V/4 A) and LT8361 (100 V/2 A) switching regulator and supports boost, SEPIC and inverting topologies. Low IQ burst mode of operation, flat switching current limit over duty cycle range, low switching losses at 2 MHz operating frequency, and a wide input range of 2.8 V to 60 V make these devices significantly better than their counterparts.
With proper demo board layout and filter design and using SSFM mode, low EMI performance can be achieved to meet CISPR 25 Class 5 EMI standards.

All devices are 16(12) MSE pin compatible, LT8362
(3 mm × 3 mm DFN(10)) and LT8364 (4 mm × 3 mm DFN(12)) are dimension compatible, simplifying design development. All devices in the LT836x family are available in E, I and H temperature grades.

Table 1. Low IQ boost/SEPIC/inverting converters; devices described in this article are highlighted.

LT8362

LT8364

LT8361

LT8330

LT8331

LT8335

Burst Mode IQ

9 µA

9 µA

9 µA

6 µA

6 µA

6 µA

Input voltage range

2.8V to 60V

2.8V to 60V

2.8V to 60V

3V to 40V

2.8V to 60V

3V to 25V

Programmable/fixed switching frequency

300 kHz to 2 MHz

300 kHz to 2 MHz

300 kHz to 2 MHz

2 MHz

100 kHz to 500 kHz

2 MHz

Spread Spectrum Frequency Modulation for Low EMI

Yes

Yes

Yes

Power switch voltage/current

60V/2A

60V/4A

100V/2A

60V/1A

140V/0.5A

28V/2A

package

3 mm × 3 mm DFN, 16(12)-pin MSE

4 mm × 3 mm DFN, 16(12) pin MSE

16 (12) pins MSE

3 mm × 2 mm DFN,
TSOT-23

16 (12) pins MSE

3mm × 2mm DFN

temperature class

E, I, H

E, I, H

E, I, H

E, I, H

E, I, H

E, I, H

Table 2. Operating modes supported by the LT836x family.

SYNC/MODE pin input

Supported working modes

(1) GND or

Burst mode of operation

(2) External clock

Pulse skip/sync

(3) 100 kΩ Resistor to GND

Burst/SSFM

(4) Floating (pin disconnected)

skip pulse

(5) INTVCC or >1.7 V

Pulse skipping/SSFM

temperature class

E, I, H

Joey Yurgelon [[email protected]

"The automotive and industrial markets require power supplies that ope…