Overview

The RT8125 (RT8125A / RT8125B) is a single-phase synchronous step-down (buck) PWM controller IC manufactured by Richtek Technology. Enclosed in a tiny WDFN-10L (3×3 mm3 \times 3\text{ mm}) or SOP-8 package, it drives external high-side and low-side N-channel power MOSFETs to convert 5V5\text{V} or 12V12\text{V} DC power rails down to low-voltage high-current outputs (0.8V…3.3V0.8\text{V} \dots 3.3\text{V}).

Operating over a supply range of 4.5V to 13.2V DC, the RT8125 features a fixed 300 kHz switching frequency, precision 0.8V0.8\text{V} internal reference voltage, adaptive shoot-through protection, loss-less over-current protection (RDS(ON)R_{DS(ON)} sensing), and thermal shutdown.

Quick reference

Input Supply Voltage (VCC / PVCC)4.5 V to 13.2 V DC
Output Voltage RangeAdjustable down to 0.8 V0.8\text{ V} DC
Reference Voltage (VFB)0.8 V0.8\text{ V} (±1.5%\pm 1.5\% accuracy)
Gate DriversIntegrated High-Side (UGATE) and Low-Side (LGATE) N-FET Gate Drivers
Switching Frequency300 kHz300\text{ kHz} fixed frequency
ProtectionOver-Voltage Protection (OVP), Under-Voltage Protection (UVP), Over-Current Protection (OCP via low-side RDS(ON)R_{DS(ON)})
PackageWDFN-10L (3×3 mm3 \times 3\text{ mm}) / SOP-8

Pinout (WDFN-10L Package)

text
                       ┌─────────────┐
                [BOOT] 1│ 1        10│ [UGATE] (High-Side Gate Out)
                [VCC]  2│            │9  [PHASE] (Phase Node Input)
                 [FB]  3│   RT8125   │8  [PVCC] (Power Supply)
                 [COMP]4│   WDFN-10  │7  [LGATE] (Low-Side Gate Out)
                 [EN]  5│            │6  [GND]
                       └─────────────┘
PinNameTypeDescription
1BOOTPowerHigh-Side Gate Driver Bootstrap Voltage Supply Input (Connect diode & cap to PHASE)
2VCCPowerController Supply Voltage Input (+4.5V to +13.2V DC)
3FBInputFeedback Sense Input (0.8V0.8\text{V} reference to GND)
4COMPOutputError Amplifier Compensation Node
5ENInputChip Enable Pin (High = Enable; Low = Shutdown)
6GNDPowerGround Reference (0 V)
7LGATEOutputSynchronous Low-Side MOSFET Gate Drive Output
8PVCCPowerMOSFET Gate Driver Supply Voltage Input
9PHASEPowerHigh-Side Driver Return & Current Sense Phase Node
10UGATEOutputHigh-Side MOSFET Gate Drive Output

Output Voltage Formula

VOUT=0.8V×(1+R1R2)V_{OUT} = 0.8\text{V} \times \left(1 + \frac{R_1}{R_2}\right)

Where R1R_1 is connected between VOUTV_{OUT} and FB, and R2R_2 is connected between FB and GND.

Common mistakes

  • Omitting the bootstrap diode & capacitor (BOOT to PHASE): The high-side N-channel MOSFET requires a gate voltage higher than VINV_{IN} to turn on. Connect a fast Schottky diode from VCC to BOOT and a 0.1 μF0.1\ \mu\text{F} ceramic capacitor between BOOT and PHASE.

Notes

  • RT8125A vs RT8125B: RT8125A includes internal soft-start and OCP latched shutdown; RT8125B features hiccup mode over-current protection.

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