The CAT4238 is a DC/DC boost converter that provides precise constant current, ideal for driving LEDs. The fixed 1 MHz switching frequency of operation allows the device to be small using external ceramic capacitors and inductors. The connected LED series drives R1 with a regulated current set by an external resistor. LED currents up to 40 mA can be supported over a wide range of input supply voltages up to 5.5 V, making the device ideal for battery-powered applications. The CAT4238 high voltage output stage is ideal for driving medium and large panel displays containing up to 10 white LEDs in series. LED dimming can be done by using a DC voltage, a logic signal or a pulse width modulation (PWM) signal. A shutdown input pin allows the device to be in a power saving mode with "zero" quiescent current. In addition to thermal protection and overload current limiting, the device also enters a very low power operating mode during "on"

LED" failure condition. The device is available in a low profile (1 mm package maximum height) 5-pin thin SOT23 package suitable for use in space

key applications.

feature

Driving High Voltage LED String Lights (38 V)

Efficiency up to 87%

Low quiescent ground current 0.6 mA

Adjustable output current

1 MHz Fixed Frequency Low Noise Operation

Soft-Start "Inrush" Current Limit

The shutdown current is less than 1 A.

Open LED Overvoltage Protection

1.9 V automatic shutdown (UVLO)

Thermal overload protection

Thin SOT23 5-pin (1 mm max height)

These devices are lead-free, halogen/BFR-free and RoHS compliant

application

GPS Navigation System

Portable Media Player

Handheld Devices, Digital Cameras

portable game console

Typical Application Circuit Diagram

Pin Description

VIN is the power supply input for the internal logic. The device is compatible with supply voltages as low as 2.8 V and up to 5.5 V. It is recommended to use a small bypass ceramic capacitor (4.7 F) placed between the VIN and GND pins near the device. If the supply voltage falls below 1.9 V, the device stops switching. SHDN is the shutdown logic input. When pin-bonded below 0.4 V, the device is in shutdown mode with almost zero current. The device is enabled if the voltage is above 1.5 V when the pin is connected to a. GND is the ground reference pin. This pin should be connected directly to the ground location on the PCB. The SW pin is connected to the drain of the internal CMOS

Power switch for the boost converter. The inductor and inductor Schottky diode anode should be connected to the SW pin. The traces going to the SW pins should be kept as short as possible to minimize loop area. An overvoltage detection circuit is connected to the SW pin. When the voltage reaches 40 V, the device enters a low power operating mode, preventing the SW voltage from exceeding the maximum rating. The FB feedback pin is adjusted to 0.3 V. Connect a resistor between the FB pin and ground to set the LED current

According to the formula:

A generation

LED 0.3V

R1

The lower LED cathode is connected to the FB pin.

block diagram

Equipment operation

The CAT4238 is a fixed frequency (1 MHz), low noise, constant current inductive boost converter with excellent line and load regulation. The device uses a high voltage CMOS power switch between the SW pin and SW ground to excite the inductor. When the switch turns off, the energy stored in the inductor is released into the load through the Schottky diode. The on/off duty cycle of the power switch is internally adjusted and controlled to maintain constant regulation. The voltage connected across the feedback resistor is 0.3 V. The feedback pin (FB). The value of the resistor sets the LED's corresponding current (0.3 V/R1). During the initial power-up phase, the duty-cycled internal power switches are limited to preventing excessive inrush currents to provide "soft-start" mode operation.

When powered by a Li-Ion battery, the device can source a load current of 20 mA into up to 10 white string LEDs. For higher input voltages, the LED current can be increased. In the event of an "open LED" failure, where

The feedback control loop becomes open and the output voltage will continue to increase. Once this voltage exceeds 40 V, the internal protection circuitry will become active and place the device into a very low power safe operating mode where only a small amount of power is delivered to the output. This is achieved by pulsing the switch every 6 seconds for about 1 second. Includes thermal overload protection circuit

Prevent equipment from operating above 150 °C in unsafe junctions. If a thermal overload condition occurs, the device automatically shuts down and resumes normal operation before the junction temperature drops to 130°C. Light Load Operation at light load conditions (below 2 mA) and input

Above 5.0 V, the CAT4238 drives 10 LEDs and the driver starts skipping pulses. Although the LED current remains well regulated, some lower frequency ripple may appear.

capacitor

The CAT4238 only requires small ceramic capacitors of 4.7 F at the input and 0.22 F at the output. Under normal conditions, a 4.7 F input capacitor is sufficient. For applications with higher output power, a 10 μF capacitor may be appropriate for larger input applications. X5R and X7R capacitor types are ideal because of their stability over temperature range.

sensor

A 47 H inductor is recommended for CAT4238 applications in most cases. Where efficiency is critical, inductors with lower series resistance are preferred. Inductors with a current rating of 300 mA or higher are recommended for most applications. The Sumida CDC5D23B-470 47 H inductor has a current rating of 490 mA and a typical series resistance (DCR) of 420 m.

Schottky diodes

The current rating of a Schottky diode must exceed the peak current flowing through it. Schottky diode performance is rated by its forward voltage at a given current. For best efficiency, this forward voltage should be as low as possible. Response Timing is also important since the driver operates at 1 MHz. The Central Semiconductor Schottky diode CMDSH05-4 (approximately 500 mA rating) is recommended for most applications.

LED current setting

The LED current is set by an external resistor R1 connected between the FEEDBACK pin (FB) and ground. The following formula gives the relationship between the resistor and the current:

R1 0.3 V

LED current

Enable LED protection

In the event of an "open LED" fault, the CAT4238 will continue to boost the output voltage to the previous maximum power

About 40 V. Once the output exceeds this level, the internal circuitry immediately puts the device into a very low power mode, limiting the total input power to about 6 mW (input current is about 1.6 mA, 3.6 V supply). The SW pin is clamped below its maximum voltage rating of 60 V. There is no need to use an external Zener diode between Vout and the FB pin. The C2 capacitor, rated at 50 V, is required to prevent any overvoltage damage to the LED in an open circuit condition.

No Zener Open LED Protection

Dimming control

There are several ways to control LED brightness.

PWM signal on SHDN pin

LED brightness dimming can be accomplished by applying a PWM signal to the SHDN input. The LED current is turned on and off repeatedly, making the average current proportional to the duty cycle. The duty cycle is 100% and SHDN is always high, corresponding to the LED at rated current. The figure below shows a 1 kHz signal with a 50% duty cycle on the SHDN pin. The recommended PWM frequency range is from 100 Hz to 2 kHz.

Filtered PWM signal

A filtered PWM signal used as a variable DC voltage can control the LED current. The figure below shows the PWM control circuit connected to the CAT4238 FB pin. The PWM signal has a voltage swing of 0 V to 2.5 V. The LED current can be dimmed from 0 mA to 20 mA. Should

The PWM signal frequency can vary from very low frequencies up to 100 kHz.