Texas Instruments UC3845D8

Overview

UC3845D8 from Texas Instruments is a current-mode PWM controller IC designed for DC-DC converter applications, featuring 8.4 V turn-on/7.6 V turn-off UVLO thresholds, up to 500 kHz oscillator frequency, 47–50% maximum duty cycle, and high-current totem-pole output capable of ±1 A peak drive. It operates across 0°C to 70°C and targets regulated-input isolated/non-isolated buck, boost, and flyback topologies.

For engineers reviewing the UC3845D8 datasheet, UC3845D8 pinout, UC3845D8 application, or UC3845D8 equivalent, key selection criteria include its narrow UVLO hysteresis (0.8 V), fixed 50% max duty cycle due to internal toggle flip-flop, 1-A gate drive capability, 5-V trimmed reference with ±2% tolerance, and SOIC-8 package compatibility with standard PCB layouts.

Technical Context

The UC3845D8 implements fixed-frequency current-mode control using an internal oscillator with RT/CT timing network, a PWM comparator that terminates switch conduction based on ISENSE voltage, and a latched logic block ensuring single-pulse per clock cycle. Its error amplifier features 90 dB open-loop gain and 1 MHz unity-gain bandwidth for stable voltage-loop compensation.

Unlike UC3842/3 variants, UC3845D8 incorporates an internal T-flipflop that halves the OUTPUT switching frequency relative to fOSC and caps duty cycle at ≤50%, making it suitable for forward or half-bridge topologies requiring symmetrical drive. The device uses VREF (5 V ±2%) to bias internal circuits and charge the timing capacitor, with COMP pin supporting both sourcing and sinking up to 10 mA for external compensation network control.

Key Specifications

Pinout & Package

UC3845D8 is housed in an 8-pin SOIC (D) package measuring 4.90 mm × 6.00 mm, with gull-wing leads and standard JEDEC MS-012AC footprint. Thermal resistance is R_θJA = 117.4°C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar current-mode PWM controller applications.

Compared with UC3844D8, UC3845D8 provides tighter UVLO hysteresis (0.8 V vs 6 V) for stable operation on clean DC rails, while TL494CDR adds flexibility at the cost of layout complexity and component count-making UC3845D8 optimal for cost-sensitive, single-output DC-DC designs requiring minimal external parts.

Availability

UC3845D8 is available at Aetrix Electronics and suitable for industrial power supplies, telecom DC-DC modules, and automotive body electronics requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for UC3845D8 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor company specializing in analog and embedded processing technologies, with leadership in power management ICs and industrial-grade controllers.

The UC384x product line was engineered for cost-effective, high-reliability current-mode control in DC-DC converters operating across commercial temperature ranges, emphasizing minimal external component count and robust fault handling.

FAQ

What is the maximum switching frequency supported by UC3845D8?

The UC3845D8 supports an oscillator frequency range of 100 kHz to 500 kHz, set by external RT and CT components. Its OUTPUT switching frequency equals half the oscillator frequency due to the internal toggle flip-flop-so maximum practical OUTPUT frequency is 250 kHz. This limit ensures reliable 47–50% duty cycle control and avoids timing violations in current-sense comparator response.

Does UC3845D8 require an external MOSFET gate driver?

No, UC3845D8 does not require an external MOSFET gate driver. Its totem-pole OUTPUT pin sources and sinks up to ±1 A peak current, enabling direct drive of N-channel MOSFETs with gate charges up to ~100 nC at 300 kHz. For higher gate charge devices, a discrete buffer may improve rise/fall times, but the UC3845D8 itself delivers sufficient drive strength for most medium-power applications.

How does the UVLO behavior of UC3845D8 differ from UC3842D8?

The UC3845D8 features 8.4 V turn-on and 7.6 V turn-off UVLO thresholds with only 0.8 V hysteresis, optimized for regulated DC inputs like 12 V or 24 V rails. In contrast, UC3842D8 uses 16 V/10 V thresholds (6 V hysteresis) for unregulated off-line supplies. This narrower hysteresis in UC3845D8 prevents oscillatory start-up near dropout but requires cleaner input voltage-confirming UC3845D8's target use case in stable DC-DC systems.

Can UC3845D8 be used in voltage-mode control configurations?

Yes, UC3845D8 can operate in voltage-mode control by applying a controlled ramp signal to the ISENSE pin, overriding its default current-sense function. The datasheet confirms this capability, and the internal PWM comparator accepts any analog input on ISENSE-including synthetic ramps for slope compensation or pure voltage-mode error injection-while retaining all other functions including UVLO, reference, and oscillator.

What is the purpose of the COMP pin on UC3845D8 and how should it be configured?

On UC3845D8, the COMP pin is the output of the internal error amplifier and serves as the control node for the PWM comparator. It must be externally compensated using a Type II or III network (typically resistor + capacitor + optional zero capacitor) between COMP and VFB or GROUND. The amplifier is internally current-limited to 10 mA, allowing forced zero-duty-cycle operation by pulling COMP to GROUND-a feature used for soft-start or fault shutdown.