LT3750 Capacitor Charge Controller IC Explained

The LT3750 is a current-mode flyback converter IC built to quickly charge large capacitors to a target voltage you can adjust. This lt3750 capacitor charge controller ic works as a dedicated capacitor charge controller for high-energy storage parts. Its main job is simple. The device controls the charge current going into the capacitor, enforces a charging limit you can program, and stops running once the capacitor reaches the set voltage. Designers like this capacitor charger because it needs very few external parts. As a capacitor charger, it sends controlled energy into the load with each switching cycle. The controller handles the whole charging profile without outside supervision. You can fully adjust both the target voltage and the charge current through external components.

Key Takeaways

• Set the target voltage using only one resistor at the RBG pin.

• Set the charge current limit by using a sense resistor at the SOURCE pin.

• The controller stops charging on its own when the capacitor reaches the set voltage.

• The LT3750 needs only a few external parts, so designs stay compact and simple.

• Use this IC for things like photoflash, beacons, and electric fences.

LT3750 Pinout Diagram and Pin Functions

LT3750 Pinout Diagram Overview

A standard lt3750 pinout diagram shows a ten-pin package. The supply, sensing, and gate drive connections are grouped in a logical order. Designers should check the datasheet drawing before layout, because the physical arrangement guides trace routing. The table below lists every pin and its role.

Pin#	Pin Name	Function
01	Vtrans	Voltage supply for the transformer primary coil and the internal boundary mode detector.
02	~Done	Open collector output. An internal NPN transistor turns on to signal the end of a charging cycle.
03	CHARGE	Logic input. A high signal starts a new cycle. A low signal shuts down the internal circuit.
04	Vcc	Voltage supply input.
05	GND	Connect directly to the main circuit ground.
06	SOURCE	Connects to the MOSFET source and the current sense resistor. The maximum current limit is 78mV/RSENSE.
07	GATE	Gate drive output for the power MOSFET.
08	RDCM	Senses discontinuous mode and restarts a cycle when transformer current reaches zero.
09	R	Converts the reflected secondary voltage into a proportional current.
10	RBG	Senses output voltage across the RBG resistor and stops charging at 1.24V.

Key Control and Sensing Pins

Three pins set the operating point. Pin 9 (R) changes the reflected secondary voltage into a current. Pin 10 (RBG) turns that current back into a voltage for the internal comparator. Charging stops when this sensed voltage equals 1.24V, so the RBG resistor sets the target level. Pin 6 (SOURCE) works with the sense resistor to set the current limit. The 78mV/RSENSE relation from Microcontrollers Lab defines the ceiling.

The remaining pins handle power and timing. Vcc and Vtrans feed the control circuit and the transformer primary. GATE drives the external MOSFET, while RDCM watches for zero transformer current. The charge pin accepts a logic high to begin a cycle and a logic low to shut the controller down. The ~Done pin reports completion through an open collector transistor.

Key Features of the Capacitor Charge Controller IC

Performance Highlights

The LT3750 capacitor charge controller IC is built for charging high-voltage capacitors using very few extra parts. It can charge to a wide range of voltages, and you set the output voltage with just one resistor. Components101 notes that the device charges to a wide range of voltages and needs only a few external parts to work. The main performance points are listed below.

• Built-in MOSFET gate driver to VCC – 2V

• Output voltage is sensed on the primary side, so no secondary-side parts are needed and isolation is easier

• VCC supply range from 3V to 24V

• Comes in a 10-lead MSOP plastic package

The patented boundary mode control scheme makes the transformer and external parts smaller. Utmel reports that a typical application can charge a 100µF capacitor to 300V in less than 300ms. These traits make the capacitor charger IC a great fit for compact, high-energy designs.

Specification	Value
Mounting Type	Surface Mount
Package / Case	10-TFSOP, 10-MSOP (0.118, 3.00mm Width)
Operating Temperature (Max.)	70°C
Number of Terminations	10
Type	Flyback, Forward Converter
Applications	Power Supply Controller

Control and Protection Features

Charge-limit control is key to safe operation. A sense resistor sets the peak primary current, and the controller stops switching on its own once charging finishes. The ~Done pin reports that event through an open collector transistor. This behavior protects the load and removes the need for outside supervision.

Primary-side sensing keeps the design simple. The output voltage is measured across the RBG resistor, and charging stops when the sensed level reaches 1.24V. The CHARGE pin takes a logic high to start a cycle and a logic low to shut the circuit down. Together, these features give the capacitor charge controller IC charging behavior that is predictable and repeatable.

How the LT3750 Capacitor Charge Controller Works

Flyback Operation and Boundary Mode Control

The LT3750 capacitor charge controller IC uses a current mode flyback converter design. In this setup, the transformer stores energy while the MOSFET switch is on. Then the switch turns off, and the stored energy moves to the secondary side and into the capacitor. A sense resistor on the primary side reports the switch current to the controller. The controller compares this signal to a reference and ends the on-time when the limit is reached.

The patented boundary mode control scheme controls the switching rhythm. Two flip-flops and fast comparators make up the timing core. One comparator watches the primary current ramp. A second comparator watches the transformer reset. The flip-flops latch each event and set the next switching state. This boundary mode control keeps the converter at the edge between continuous and discontinuous operation. The Engineering Knowledge reports that this design cuts transition losses and controls transformer size. The same source notes a typical use case: charging a 100µF capacitor to 300V in less than 300ms.

Timing accuracy matters here. An unwanted spike on the current sense line can trip a comparator too early. The switch then turns off before the current reaches its true limit. The converter loses its boundary mode rhythm, and proper operation stops. Careful layout of the sense trace and a small filter at the SOURCE pin prevent this failure.

Programming Charge Voltage and Current

Two external resistors set the operating point. The RBG resistor programs the target output capacitor voltage. The R resistor turns the reflected secondary voltage into a proportional current. Charging stops when the sensed level at RBG reaches 1.24V. The target voltage therefore scales with the RBG value, and the datasheet ratio sets the exact level.

The sense resistor at the SOURCE pin sets the current limit. The relation is peak current limit = 78mV / RSENSE. A smaller resistor raises the limit; a larger resistor lowers it. This peak value applies to the primary switch current, not the average output current. Designers should pick the resistor size for the peak they want and confirm the transformer can handle it.

The CHARGE pin starts and stops operation. A logic high begins a cycle, and a logic low shuts the controller down. The ~Done pin reports completion through an open collector transistor. Together, these settings give the designer full command of the charge profile with very few external parts.

Practical Usage and LT3750 Applications

Charging Time Calculation

A common example circuit comes from the LT3750 datasheet. It shows a 9A, 300V charger built around this part. The design uses ceramic and electrolytic decoupling capacitors on the supply rails.

Charging time follows a simple formula. The time equals capacitance times the target voltage, divided by the charge current. Keep the units consistent: capacitance in farads, voltage in volts, current in amps, and the result in seconds. This formula gives an average number. It assumes the current stays constant, so treat the result as an estimate, not a guarantee.

Try the formula on a 100µF capacitor charged to 300V at 9A. The stored charge equals 0.0001F times 300V, or 0.03 coulombs. Dividing by 9A gives about 3.3ms. Real hardware will take longer, because the current drops near the end of the cycle and losses use up some energy. The datasheet example still shows how fast this capacitor charger moves energy into a large load.

Check three things before you trust a calculated time. First, make sure the transformer can deliver the peak current the sense resistor allows. Second, confirm the supply can provide that current without dropping. Third, leave some margin for efficiency. A real capacitor charging circuit often needs more time than the ideal math predicts.

LT3750 Applications

The part fits designs that need a small, self-contained high voltage capacitor charger. Common uses include:

• Emergency warning beacons

• Professional photoflash systems

• Security and inventory control systems

• High voltage power supplies

• Electric fences

• Detonators

Each use shares a common idea. A capacitor stores energy over time, then releases it in a short burst. The controller manages the slow charge phase and stops cleanly at the target voltage. Photoflash systems benefit from the fast cycle and the automatic stop. Beacons and electric fences value the low part count and the simple logic interface. Detonators and pulsed power designs need repeatable charging, and the primary-side sensing supports that goal.

These lt3750 applications suit engineers who want a dedicated capacitor charge controller with few external parts. The device handles charging capacitors of many sizes. It regulates the output through the RBG resistor and enforces the current limit through the sense resistor. A designer can therefore tune one board for several energy levels with a resistor change. That flexibility keeps the part useful across photoflash, beacons, and pulsed power work.

The LT3750 is a current-mode flyback capacitor charge controller with boundary mode control. It controls the current going into a large capacitor and stops when the target level is reached. The lt3750 capacitor charge controller ic lets designers set the target voltage with one resistor. Charge-limit control stops charging on its own, and the ~Done pin reports when charging is complete. It needs only a few external parts, so the capacitor charger works well for high-voltage jobs. The output stays predictable across many capacitor sizes. This controller works best in designs that need compact, repeatable energy storage, such as photoflash, beacons, and pulsed power. Keep the LT3750 in mind when a project needs simple, self-contained high-voltage charging.

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

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