LT3750 Capacitor Charge Controller

The LT3750 is a current mode flyback converter that is optimized to charge high voltage capacitors quickly and efficiently using a minimum number of external components.

 

LT3750 Pin Configuration

Pin Number	Pin Name	Pin Description
1	VTRANS	This pin connects to the transformer supply voltage and is internally connected to the boundary mode detection circuitry.
2		This is an open collector pin that signals the completion of one charging cycle.
3	CHARGE	Bringing this pin high will start the charging cycle. Internally connected to a one-shot, so a single pulse will trigger the chip too.
4	VCC	Chip power supply voltage. May or may not be VTRANS.
5	GND	Chip ground.
6	SOURCE	Connected to the source of the power MOSFET and the current sense resistor.
7	GATE	Gate drive output for the power MOSFET.
8	RDCM	Senses boundary mode through an appropriate resistor to begin a new switching cycle.
9	RVOUT	Converts the reflected secondary voltage to a current.
10	RBG	Converts the current from pin 9 to a voltage that is detected by a comparator to stop charging.

 

LT3750 Features

• Charges to a wide range of voltages
• Output voltage adjusted using a single resistor
• Integrated MOSFET gate driver to V_CC – 2V
• Output voltage sensed on the primary side, no secondary side components needed – isolation is easier
• V_CC from 3V to 24V
• Minimum external parts
• Available in a 10-lead MSOP plastic package

 

Note: More technical details of this IC can be found in the LT3750 Datasheet linked at the bottom of this page.

 

Equivalent for LT3750: LT3751

Other capacitor charge controller ICs: LT3751

 

Where to Use LT3750

Anyone who has tried to build a high voltage capacitor charger knows how difficult this task is, especially when developing a circuit from scratch. The end results often show poor efficiency and charge times. Why bother with all the fuss when a single chip and a handful of discrete parts can do the job for you?

 

LT3750 integrates all the functions of a capacitor charging circuit into a small 10-pin chip – voltage detection, MOSFET driving, and control logic.

 

How to Use LT3750

The LT3750 is a simple implementation of a flyback converter without regulation (in the traditional sense) since it is a capacitor charger.

 

Flyback converters take in discrete packets of energy from one power source, store them in a coupled inductor (‘transformer’), and send the energy at a different voltage to the secondary.

 

The LT3750 operates in current mode, meaning that the peak primary current is constant in every switching cycle. The peak current can be set using a resistor. The value of the resistor can be calculated using this formula:

RSENSE = 78mV / IPEAK

 

After the current limit is reached, the MOSFET is turned off and the energy stored in the primary is transferred to the secondary. During this time, the secondary voltage (divided by the turns ratio) is reflected on the primary, which is seen as an increase in voltage at the drain node when compared to the supply. The LT3750 detects this difference, which is proportional to the output voltage, through the RBG and RV_OUT resistors. The output voltage is given by the formula:

VOUT = 1.24V * (RVOUT/RBG) * N

Where N is the turns ratio. The datasheet recommends fixing the value of the RBG resistor to 2.49K, preferably with a tolerance of 1%.

 

Once the secondary energy transfer is complete, the reflected voltage on the primary drops. The RDCM resistor detects this and starts a new switching cycle. Again, the recommended value for the RDCM resistor is 43K at 5%. Here is a circuit from the datasheet, showing a 9A 300V charger.

For the selection of parts, both ceramic and electrolytic decoupling is recommended. Though custom transformers will work, Coilcraft’s DA203X series is highly recommended.

 

PCB Layout Tips

Keep the discrete components close to the chip. A full ground plane is a must. The gate drive loop must be kept small.

 

Applications of LT3750

• Emergency Warning Beacons
• Professional Photoflash Systems
• Security/Inventory Control Systems
• High Voltage Power Supply
• Electric Fences
• Detonators