Illuminating LED Driver Design

By Hans Brueggemann

Page Contents:   [ Featured Products | Datasheets | Application Notes | Buy Now ]

Incorporating high-power LEDs into lighting design brings with it some exciting opportunities. But it is not without challenges, especially where the power supply is concerned. This article explains how to design efficient driver circuits.

Read this to learn about The challenges faced when developing a power supply for an LED The different kinds of LED-driver circuitry in common use Guidelines to consider when designing LED-driver circuitry

High power LEDs are transforming the lighting industry and revolutionizing products across a range of application areas, from handheld flashlights, to residential lighting. One potential limitation they present is the need for a more complex power supply, compared to filament bulbs or fluorescent tubes. However, if the right steps are taken, LED driver design does not need to be a headache, as this article aims to explain. This article will outline a systematic assessment and decision process that should help engineers to assemble the right driver for an LED application. High power LEDs are those devices operating at 1W and above, such as the LUXEON® LEDs from Philips Lumileds.

 

 

Start with a Specification
Once the optical requirements have been defined, it is advisable to detail the application parameters that will influence the driver design. These include:

• The type of application: Is it a portable, automotive or mains-powered design, for example?
• Power supply source related issues: Are there any constraints with respect to the minimum/maximum input voltage, battery types, safety regulations and power quality?
• Power supply sink related issue: What are the requirements in terms of the output current (I_OUT) and voltage (V_OUT)?
• Features: Is there a requirement for capabilities such as protection against over/undervoltage, battery reversal, over temperature or output short-circuit, for example?

 

 

Driver Circuits for Portable Applications
There are many standard driver configurations to choose from depending on the requirements present. Fig. 1 illustrates the main elements of a typical lighting driver. These comprise the power source (V₁) power converter and power sink (D₁) in addition to a current sensing device (R₁).

 

 

A driver circuit for a handheld flashlight is a good illustrative example. An approximate calculation for battery capacity
(C_batt) is:

Cbatt = (VOUT x IOUT)/((etaconv x VIN) x T)

Where: T = target operating time

etaconv = conversion efficiency

 

The etac_conv typically ranges from 0.8 to 0.92 for switched-mode drivers. After C_batt is determined, the following guidelines will aid the calculation of battery voltage:

• The highest efficiency is achieved by matching V_IN to V_OUT as closely as possible.
• Transformer-less converters are the simplest solution if all that is required is to step-up or step-down V_IN to V_OUT. For a step-up, or boost, converter, V_IN(MAX) should be lower than V_OUT. For a step-down, or buck, V_IN(MIN) should be higher than V_OUT.
• V_IN/V_OUT ratios over four and under 0.25 impact the efficiency of transformer-less converters by up to 10 or 20 percent compared to a converter with a ratio of two.
• Converters catering for input voltages in excess of 5.5V are based on more expensive technology than those of equal power rating but lower input voltages.

 

Using a single high power LED in conjunction with a current sense resistor (R₁) the required output voltage for the circuit in Fig. 1 would be:

VOUT = 3.5V + 1.25V = 4.75V

 

Taking V_dchg and V_chg to be the lower and upper voltages of a single battery cell, with values of 0.95V and 1.4V, respectively, the number of serial connected cells (N) can be calculated as:

Ndchg = VOUT/Vdchg = 4.75/0.95 = 5 cells

 

When nearing full discharge, the corresponding value for the upper voltage level is:

Vchg = VOUT/Vchg = 4.75/1.4 = 3.4

 

(i.e. four cells), when fully charged with nominal voltage of the battery cell (V_nom) at 1.2V, the nominal V_IN will be:

VIN = 4 x 1.2V = 4.8V

and:

VIN/VOUT = 1.01

 

This a good match between V_IN and V_OUT, making it easier to achieve a high level of efficiency. However in this case:

VIN(MIN) = 4 x 0.95 = 3.8V

 

This is lower than the value for V_OUT of 4.75V, while:

VIN(MAX) = 4 x 1.4 = 5.6V

 

which is higher than V_OUT. Therefore, the four-cell solution should be avoided, unless V_OUT can be made lower than V_IN(MIN). Otherwise, a more complex buck-boost topology would be needed.

Choosing five cells in series yields:

VIN = 5 x 1.2V = 6.0V

and:

VIN/VOUT = 1.26

 

Voltage pre-match is still good, and both V_IN(MIN) and V_IN(MAX) are greater than V_OUT allowing the use of a simple buck converter.

Often the designer may be forced to design the driver circuit around a defined battery capacity. Here the choice of driver circuit is determined by the ratio V_IN/ V_OUT.

If V_IN is always higher than V_OUT, then a resistor, Low Drop-Out regulator (LDO) or buck converter could be used to step-down the voltage to the required level. Surprisingly, a simple resistor may be the 'converter'of choice, if V_IN and V_OUT are close in value, with V_IN always slightly higher than V_OUT. This will also minimize the converter costs.

A slightly better solution would be to use an LDO. Although more expensive and of similar efficiency in the likely range of V_IN(MIN), an LDO is the simplest way to implement precise current control and at the same time allow for Pulse-Width Modulated (PWM) dimming.

The LDO also wastes less battery power at V_IN(MAX) compared to a resistor. Being simple and easy to implement, both resistors and LDOs enjoy broad usage in low-power lighting applications such as mini flashlights and toys. However, when driving high power LEDs, these approaches are somewhat inefficient. A buck converter is a better choice. While being more expensive, the buck converter incorporates all the features of an LDO with higher conversion efficiency over a given range of V_IN, extending battery life.

If V_IN is always lower than V_OUT, a boost converter is the only alternative. These function well with input voltages as low as 0.7V. The Sipex SP6648, for example, can start up with an input voltage of 1.1V, allowing a single cell to efficiently power an LED.

 

 

Coping with Varying Input Voltage
There are cases when V_IN point variations can be either higher or lower than V_OUT, depending on battery capacity. There are several converter topologies available to cope with this situation. Of these, buck-boost, SEPIC and Cuk are the most widely used.

The traditional buck-boost is the most cost effective, because it uses a single inductor, while other types of converters typically use two. There is a price to pay, however, because both the input and output terminals can be noisy due to the non-continuous current flow.

The SEPIC has two major advantages over the buck-boost converter. First, input noise is relatively low due to the primary inductor. Second, because the converter employs an additional decoupling capacitor, there is no direct connection between input and output, effectively protecting the LED from over voltage in the case of a converter fault. A circuit is shown in Fig.2.

 

 

The Cuk converter is similar to the SEPIC, featuring low input noise and a decoupling capacitor. However, a disadvantage is the reversed polarity on V_OUT. This means the circuit requires an additional op-amp to measure the output current. To function effectively, the op-amp requires sufficient gain-bandwidth and rail-to-rail input range. The Microchip MCP6041 is recommended.

 

 

Improving Battery Life Using Current Sensing
Because most adjustable switched-mode converters and controllers are designed for voltage-source applications, rather than the current-source mode required to drive high-power LEDs, their feedback inputs require 1.25V. Providing this voltage is the function of R1 in the Fig. 1 circuit. In this configuration, the shunt resistor dissipates 0.4W of heat.

This 0.4W is a serious drain on battery life; with five AA cells, and a 1500mAh total capacity, the theoretical operating time is nine hours. This is reduced to 5.8 hours when the power used by the shunt resistor is added.

It is possible to reduce the shunt voltage from 1.25V to, say, 100mV to improve efficiency, using additional sensing and shunt-voltage amplification. This can be done with an op-amp configuration in a circuit similar to that shown for the Cuk circuit, as in Fig. 3.

 

 

Another way to sense the LED current is to use a dedicated current sensor, such as the Zetex ZXCT1009. This solution reduces the part count and provides an option of 'floating' current sensing. In other words, the shunt resistor no longer has to be referred to circuit ground. This is particularly helpful when lower priced boost controllers are driving the LEDs in a buck derived topology.

For both these examples, the shunt resistor power consumption is reduced from 0.4W to 35mW, boosting the battery lifetime from 5.8 to 7.8 hours. Alternatively, battery life can be kept to five hours but the number of cells reduced from five to four.

A third option is to dispense with current sensing on the output side by using a converter with internal current limiting.

Normally internal current limiting works with reference to a fixed value, decreasing the output current through the LED when the input voltage decreases and vice versa. However, if the converter's current limiting threshold can be programmed, it is possible to feed forward a portion of the input voltage into the current limiting setpoint. This 'steers' the current limit such that the output current remains in a tolerance band 15% above the full input-voltage-variation band.

 

 

Fig. 4 shows an example of this arrangement for a handheld flashlight, in this case using a Sipex SP6648 boost converter. The efficiency is approximately 92%. The output voltage is set to 5V, slightly above the minimum recommended level but this is necessary to maintain the controller in current limiting mode.

For single-cell operation, the Zetex ZXSC300 in boost mode provides a high-efficiency solution. Its 20mV current sense threshold voltage only requires a low value sense resistor and consequently minimizes power loss.

There are many challenges to address when developing driver circuitry for high power LEDs. However, once the key design constraints have been established, such as the need for portability or particular protection requirements, the options for the designer are well sign-posted. It should be relatively straightforward to manage the trade-offs between factors such as size, cost and efficiency to arrive at a power supply design that is a good fit with the application.

 

 

 

SP6648 - Ultra-low Quiescent Current, High Efficiency Boost Regulator

 

Features Ultra-low 12µA quiescent current 400mA output current at 2.6V Input: 3.3V OUT 94% efficiency from 2 cell to 3.3V OUT Wide input voltage range: 0.95V to 4.5V 3.3V fixed or adjustable output Integrated synchronous rectifier: 0.3Ω 0.3Ω switch Anti-ringing switch technology Programmable Inductor Peak Current Logic shutdown control Under voltage lock-out at 0.61V Programmable low battery detect Single or dual cell alkaline Small 10 pin DFN package and industry standard 10 pin MSOP

 

 

 

The MCP6041 operational amplifier (op-amp) has a gain bandwidth product of 14kHz with a low typical operating current of 600nA and an offset voltage that is less than 3mV. The MCP6041 uses Microchip's advanced CMOS technology, which provides low bias current, high speed operation, high open-loop gain and rail-to-rail output swing. The MCP6041 operates with a single supply voltage that can be as low as 1.4V, while drawing less than 1.0 of quiescent current. The MCP6041 is available in standard 8-lead PDIP, SOIC and MSOP packages. This amplifier is ideal for industrial process control, low power battery operated devices, portable equipment and wearable products.

 

Parameter Name Value Temp. Range (°C) -40°C to +85°C Vos Max (mV) 3 Offset Voltage 0 Iq Typical (µA) 0.6 Iq Max (mA) 0.001 GBWP (kHz) 14 Operating Voltage Range (V) 1.4 - 5.5 Input Voltage Noise Density (nV/rt(Hz)) 170 PSRR Min (dB) 70 CMRR Min (dB) 62 ΔVos/ΔTa (uV/°C) 1.5 Ib (pA) 1 Aol (dB) 115 PM (deg) 65 Isc (mA) 21 Rail-to-Rail In/Out

 

 

 

ZXCT1009
ZXCT current monitors with a current output convert a high side current measurement to a ground referred current output. This greatly simplifies high sided current measurements. The devices enable gain to be set by a single external transistor. The common-mode voltage of the monitored current can be substantially increased by the addition of 1 or 2 transistors.

 

Features Accurate high side current sensing Output voltage scaling Up to 2.5V sense voltage 2.2 - 20V supply range 1% typical accuracy

Applications Battery chargers Smart battery packs DC motor control Over current monitor Power management

 

 

ZXSC300
The ZXSC300 is a single or multi cell LED driver designed for applications where step-up voltage conversion from very low input voltages is required. These applications mainly operate from single 1.5V or 1.2V battery cells. The circuit generates constant current pulses that are ideal for driving single or multiple LED’s over a wide range of operating voltages.

 

Features 94% efficiency Minimum operating input voltage 0.8V Fixed output current Low saturation voltage switching transistor SOT23-5 package

Applications LED flashlights and torches LED backlights White LED driving Multiple LED driving Solar equipment

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
MCP6041T-I/OT		600nA, Rail-to-Rail Input/Output Op-Amp
MCP6041T-I/SN		600nA, Rail-to-Rail Input/Output Op-Amp
SP6648EB		Evaluation Board
SP6648ER-L		Ultra-low Quiescent Current, High Efficiency Boost Regulator
SP6648ER-L/TR		Ultra-low Quiescent Current, High Efficiency Boost Regulator
SP6648EU		Ultra-low Quiescent Current, High Efficiency Boost Regulator
SP6648EU/TR		Ultra-low Quiescent Current, High Efficiency Boost Regulator
SP6648EU-L		Ultra-low Quiescent Current, High Efficiency Boost Regulator
SP6648EU-L/TR		Ultra-low Quiescent Current, High Efficiency Boost Regulator
ZXCT1009FTA		High Side Current Monitor
ZXSC300E5TA		LED Driver Controller

refers to New Product Introduction