Overview

The CAT4238 (CAT4238TD-GT3) is a high-voltage step-up (boost) DC-DC LED driver IC manufactured by ON Semiconductor. Operating at a fixed switching frequency of 1.0 MHz, it generates output voltages up to 38 Volts38\text{ Volts} to drive up to 10 series-connected white LEDs at constant currents up to 30 mA30\text{ mA}.

Powered from single-cell Li-ion batteries or regulated 5V power rails (2.8V2.8\text{V} to 5.5V5.5\text{V}), the CAT4238 features a low 300 mV300\text{ mV} internal feedback reference voltage, minimizing power loss across the current-sense resistor and achieving efficiency up to 87%87\%.

Quick reference

Input Voltage (VIN)2.8 V to 5.5 V DC
Output Voltage Limit (VOUT)Up to 38.0 V38.0\text{ V} DC (Internal open-LED clamp protection at 38V)
LED CapacityUp to 10 Series-Connected White LEDs
Max LED Current30 mA30\text{ mA} continuous (Set via sense resistor RSETR_{SET})
Feedback Voltage (VFB)300 mV300\text{ mV} reference
Switching Frequency1.0 MHz1.0\text{ MHz} fixed PWM frequency
Dimming ModesDirect PWM Dimming or DC Voltage Analog Dimming via SHDN
PackageTSOT-23 6-lead (SOT-23-6)

Pinout (TSOT-23-6 Package)

text
             ┌───┴───┐
        SW  1│ 1   6 │ GND
       GND  2│       │ 5 SHDN
        FB  3│CAT4238│ 4 VIN
             └───────┘
PinNameTypeDescription
1SWOutputInternal Power MOSFET Drain Connection (Connect to Inductor & Schottky Diode)
2GNDPowerGround Reference (0 V)
3FBInputFeedback Current Sense Input (300 mV300\text{ mV} reference to GND)
4VINPowerPower Supply Input (+2.8V to +5.5V DC)
5SHDNInputShutdown / PWM Dimming Control (High = Enable; Low = Shutdown <1μA< 1\mu\text{A})
6GNDPowerGround Reference (0 V)

LED Current Programming Formula

The LED string current (ILEDI_{LED}) is set by resistor RSETR_{SET} connected between FB (Pin 3) and GND:

ILED=VFBRSET=300 mVRSET=0.30VRSETI_{LED} = \frac{V_{FB}}{R_{SET}} = \frac{300\text{ mV}}{R_{SET}} = \frac{0.30\text{V}}{R_{SET}}

For ILED=20 mA  ⟹  RSET=0.30V0.020A=15 Ω.\text{For } I_{LED} = 20\text{ mA} \implies R_{SET} = \frac{0.30\text{V}}{0.020\text{A}} = 15\ \Omega.

Common mistakes

  • Operating with an open LED load: Disconnecting the LED string while powered causes the boost converter output voltage to rise rapidly. The CAT4238 includes an internal 38V38\text{V} overvoltage protection clamp, but omitting output capacitors leads to voltage spikes.
  • Using a low-frequency inductor: The 1.0 MHz switching frequency requires a 10 μH…22 μH10\ \mu\text{H} \dots 22\ \mu\text{H} low-DCR surface-mount power inductor rated for ≥350 mA\ge 350\text{ mA} saturation current.

Notes

  • CAT4238 vs AP3012: CAT4238 features an integrated 38V open-LED protection clamp; AP3012 requires an external Zener diode for overvoltage protection.

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