Combined voltage and current control loops simplify LED Drivers … and many other applications

Combined voltage and current control loops simplify LED Drivers … and many other applications

Technology News |
By eeNews Europe

Consider, for instance, the challenge of charging a supercapacitor in a minimum amount of time from a power-limited supply. To maintain constant input power, the controlled charging current must decrease as the output (supercapacitor) voltage increases. The LT3796 solves the problem of power limited or constant current/constant voltage regulation by seamlessly combining a current regulation loop and two voltage regulation loops to control an external N-channel power switch. The inherent wired-OR behavior of its three transconductance error amplifiers summed into the compensation pin, VC, ensures that the correct loop (that is, the one closest to regulation) dominates.

The additional, standalone current sense amplifier can be configured for any number of functions, including input current limit and input voltage regulation.

The LT3796’s wide VIN range (6V to 100V) and rail-to-rail (0V to 100V) output current monitoring and regulation allow it to be used in a wide variety of applications from solar battery chargers to high power LED lighting systems. The fixed switching frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. The LT3796 incorporates a high side current sense, enabling its use in boost, buck, buck-boost or SEPIC and flyback topologies.

High Power LED Driver with Robust Output Short Circuit Protection


Figure 1. A 34W LED driver with robust output short-circuit protection.For full resoultion click here.

Figure 1 shows the LT3796 configured as a boost converter to drive a 34W LED string from a wide input range. The LED current is derated at low input voltages to prevent external power components from overheating. The front-end current sense amplifier monitors the input current by converting the input current to a voltage signal at the CSOUT pin with

The resistor network at the FB1 pin provides OPENLED protection, which limits the output voltage and prevents the ISP pin, ISN pin and several external components from exceeding their maximum rating. If an LED fails open or if the LED string is removed from the high power driver, the FB constant voltage loop takes over and regulates the output to 92.5V. The VMODE flag is also asserted to indicate an OPENLED event.

The LT3796 includes short-circuit protection independent of the LED current sense. The short-circuit protection feature prevents the development of excessive switching currents and protects the power components. The protection threshold (375mV, typ) is designed to be 50% higher than the default LED current sense threshold.

Once the LED overcurrent is detected, the GATE pin drives to GND to stop switching, the TG pin is pulled high to disconnect the LED array from the power path and the FAULT pin is asserted. The Schottky diode D2 is added to protect the drain of PMOS M2 from swinging well below ground when shorting to ground through a long cable. The PNP helper Q1 is included to further limit the transient short-circuit current.

If there is no resistor between the SS pin and VREF pin, the converter enters hiccup mode and periodically retries as shown in the Figure 2. If a resistor is placed between VREF and SS pin to hold SS pin higher than 0.2V during LED short, then the LT3796 enters latchoff mode with GATE pin low and TG pin high, as shown in Figure 3. To exit latchoff mode, the EN/UVLO pin must be toggled low to high.

Figure 2. Short LED protection: hiccup mode (without R11 in Figure 1)

LED Driver with High PWM Dimming Ratio

Using an input referred LED string allows the LT3796 to act as a buck mode controller as shown in Figure 4. The 1MHz operating frequency enables a high PWM dimming ratio. The OPENLED regulation voltage is set to

through the independent current sense amplifier at CSP, CSN and CSOUT pins. During the PWM off phase, the LT3796 disables all internal loads to the VC pin and preserves the charge state. It also turns off the PMOS switch M2 to disconnect the LED string from the power path and prevent the output capacitor from discharging. These features combine to greatly improve the LED current recovery time when PWM signal goes high. Even with a 100Hz PWM input signal, this buck mode LED driver can achieve a 3000:1 dimming ratio as illustrated in Figure 5.

Figure 5. 3000:1 PWM dimming ratio of the circuit in Figure 4 at VIN = 24V and PWM = 100Hz

SEPIC Converter with RWIRE Compensation

Voltage drops in wiring and cables can cause load regulation errors. These errors can be corrected by adding remote sensing wires, but adding wires is not an option in some applications. As an alternative, the LT3796 can adjust for wiring drops, regardless of load current, provided that the parasitic wiring or cable impedance is known.

Figure 6. This SEPIC converter compensates for voltage drops in the wire between the controller and the load (RWIRE)

Figure 6 shows a 12V SEPIC converter that uses the RWIRE compensation feature. RSNS1 is selected to have 1A load current limit controlled by the ISP, ISN pins. The resistor network R1–R5, along with the LT3796’s integrated current sense amplifier (CSAMP in Figure 7), adjusts the OUT node voltage (VOUT) to account for voltage drops with respect to the load current. This ensures that VLOAD remains constant at 12V throughout the load range.

Figure 7 shows how the LT3796’s internal CSAMP circuit plays into the operation. The LT3796’s voltage loop regulates the FB1 pin at 1.25V so that I3 stays fixed at 100µA for R5 = 12.4k. In Figure 7, VOUT changes with current I2 as VOUT = 1.25V + I2 • R4. If the change of I2 • R4 can offset the change of ILOAD • (RSNS1 + RWIRE), then VLOAD will stay constant.

Figure 7. RWIRE voltage drops are compensated for via the LT3796’s CSAMP circuit

Referring to Figure 7, the divider R1/R3 from VOUT sets the voltage regulated at CSP by the current I1 flowing in R2. I1 is conveyed to the FB1 node where it sums with I2.

As the output current increases, I1 decreases due to the increasing voltage drop across RSNS; its decrease must be compensated by a matching increase in the current I2 to maintain the constant 100µA into FB2. This increase in I2 with output current is what gives VOUT the positive load regulation characteristic. The positive load regulation is just what is needed to compensate for the cable drop.

The measured VLOAD and VOUT with respect to ILOAD are shown in Figure 8. Clearly, VLOAD is independent of ILOAD when ILOAD is less than the 1A current limit. When ILOAD approaches 1A, the current loop at ISP and ISN pins begins to interfere with the voltage loop and drags the output voltage down correspondingly. The load transient response is shown in Figure 9.

Figure 9. Load step response of the circuit in Figure 6

Solar Panel Battery Charger

Solar powered devices rely on a highly variable energy source, so for a device to be useful at all times, energy from solar cells must be stored in a rechargeable battery. Solar panels have a maximum power point, a relatively fixed voltage at which the panel can produce the most power. Maximum power point tracking (MPPT) is usually achieved by limiting a converter’s output current to keep the panel voltage from straying from this value. The LT3796’s unique combination of current and voltage loops make it an ideal MPPT battery charger solution.

Figure 10 shows a solar panel to sealed lead acid (SLA) battery charger driven by the LT3796. The charger uses a three-stage charging scheme. The first stage is a constant current charge. Once the battery is charged up to 14.35V, the charging current begins to decrease. Finally, when the required battery charge current falls below 100mA, the built-in C/10 termination disables the charge circuit by pulling down VMODE, and the charger enters float charge stage with VFLOAT = 13.5V to compensate for the loss caused by self-discharge.

Figure 10. A solar panel battery charger maximum power point tracking (MPPT)

The charging current is programmed by the resistor network at the CSP and CSOUT (CTRL) pins as follows,

Maximum power point tracking is implemented by controlling the maximum output charge current. Charge current is reduced as the voltage on the solar panel output falls toward 28V, which corresponds to 1.1V on the CTRL pin and full charging current, as shown in Figure 11. This servo loop thus acts to dynamically reduce the power requirements of the charger system to the maximum power that the panel can provide, maintaining solar panel power utilization close to 100%.

Supercapacitor Charger with Input Current Limit

Supercapacitors are rapidly replacing batteries in a number of applications from rapid-charge power cells for cordless tools to short term backup systems for microprocessors. Supercapacitors are longer lasting, greener, higher performance and less expensive over the long run, but charging supercapacitors requires precise control of charging current and voltage limiting to prevent any system-wide damage or damage to the supercapacitor.

Some applications require that the input current is limited to prevent the input supply from crashing. Figure 12 shows a 1.67A supercapacitor charger with 28V regulated output voltage and 1.33A input current limit. The input current is sensed by RSNS1, converted to a voltage signal and fed to the FB2 pin to provide input current limit.

Figure 12. A 28V/1.67A supercapacitor charger with input current limit

In each charging cycle, the supercapacitor is charged from 0V. The feedback loop from VOUT to the RT pin through R3, C5, R5, R10, R4, and Q1 to RT works as frequency foldback to keep regulation under control. In Figure 13, the input current and output charging current are plotted against output voltage for this charger, showing the LT3796 maintaining the output current regulation until the input current moves close to the 1.33A input current limit.


The LT3796 is a versatile step-up DC/DC controller that combines accurate current and voltage regulation loops. Its unique combination of a single current loop and two voltage loops makes it easy to solve the problems posed by applications that require multiple control loops, such as LED drivers, battery or supercapacitor chargers, MPPT solar battery chargers, and step-up or SEPIC converters with input and output current limit. It also includes a number of fault protection and reporting functions, a top gate driver and current loop reporting.

The LT3796 in a 28-lead TSSOP package performs tasks that would other­wise require a number of control ICs and systems. It offers a reliable power system with simplicity, reduced cost and small solution size.

About the author:

Xin (Shin) Qi is a Senior Design Engineer at Linear Technology in the Power Product group. He joined the company in 2005 and has over 8 years of experience designing DC-DC converters for a variety of applications in the industrial, automotive and medical markets. He holds a PHD from the University of Florida, an MSEE from Shanghai Jiao Tong University and a BSEE from Northeastern University.

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