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Home > Products > Integrated Circuits (ICs) > Power Management (PMIC) > AC DC Converters, Offline Switches

STMicroelectronics L5991D

Name: L5991D
Brand: STMicroelectronics
SKU: L5991D
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: L5991D

Manufacturer:: STMicroelectronics

Category:: AC DC Converters, Offline Switches

Package:: 16-SOIC (0.154", 3.90mm Width)

Datasheet:: L5991D.pdf

Description:: IC OFFLINE SWITCH MULT TOP 16SO

Quantity:: RFQ Add To RFQ list

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Product details

Overview

L5991D from STMicroelectronics is a primary-side current-mode PWM controller IC for offline and DC-DC flyback power supplies, featuring 1 MHz switching frequency, <120 µA start-up current, 1 A peak output drive, programmable 0–100% duty cycle, and integrated standby mode with oscillator frequency reduction under light load. It implements precise overcurrent protection with hiccup-mode restart, internal 100 ns leading-edge blanking, and IN/OUT synchronization capability.

For engineers reviewing the L5991D datasheet, L5991D pinout, L5991D application, or L5991D equivalent, this controller is selected for high-efficiency AC-DC adapters, auxiliary power supplies in industrial SMPS, and low-noise isolated DC-DC converters requiring stable duty-cycle control, soft-start sequencing, and robust fault recovery.

Technical Context

This IC uses BCD60II process technology to integrate a precision 5.0 V ±1.5% reference, error amplifier with 90 dB open-loop gain and 4 MHz unity-gain bandwidth, and a dual-threshold standby comparator (VT1 = 2.5 V, VT2 = 4.0 V) that dynamically scales oscillator frequency between f_osc and f_SB based on V_COMP level. The oscillator supports external timing via RCT pin with separate RA/RB configuration for normal and standby frequencies.

It features fully latched PWM logic with double-pulse suppression, programmable maximum duty cycle (via DC pin voltage), and pulse-by-pulse current limiting with 1.2 V fault threshold at ISEN. The 1 A totem-pole output drives N-channel MOSFETs directly, while internal UVLO (14–16 V turn-on, 4.5 V hysteresis) ensures reliable start-up and brown-out protection.

Key Specifications

Parameter	Value and Actual Design Meaning
Switching Frequency	Up to 1 MHz; externally programmable via RCT pin using CT, RA, RB; standby frequency selectable independently.
Start-up Current	<120 µA at V_CC = V_CCON − 0.5 V; minimizes losses in high-resistance start-up networks.
Output Drive Current	1 A peak source/sink; sufficient to directly drive power MOSFET gates without external buffers.
V_REF Accuracy	5.0 V ±1.5% at 25°C; stable over temperature (±0.4 mV/°C), supports precision feedback and timing circuits.
Current Sense Threshold	1.2 V typical; triggers overcurrent protection with 100 ns leading-edge blanking to reject switching noise.
Standby Thresholds	V_COMP falling = 2.5 V (VT1), rising = 4.0 V (VT2); enables automatic frequency scaling without external circuitry.
Duty Cycle Range	0–100% programmable via DC pin (1–3 V input); 50% limit enabled by connecting DC-LIM to V_REF.
UVLO Hysteresis	4.5 V (L5991), 0.8 V (L5991A); prevents oscillation near turn-off threshold during line dips.

Pinout & Package

Package: SO16 (Small Outline, 16-pin, 1.27 mm pitch, body width 3.9 mm).

Pin/Terminal	Circuit Role	Design Meaning
1 SYNC	In/Out synchronization interface	Master: outputs 4 V, 7 mA sync pulses; Slave: edge-triggered input with 1 V low / 3.5 V high thresholds.
2 RCT	Oscillator timing node	Connects external CT, RA, RB; sets f_osc (RA//RB) and f_SB (RA only) with KT compensation.
3 DC	Duty cycle control input	0–3 V input sets max duty cycle (0–100%); <1 V disables output; internal pull-up if floating.
4 V_REF	5.0 V reference output	±1.5% accuracy; supplies external circuits; requires 0.1 µF bypass to SGND for stability.
5 V_FB	Error amp inverting input	Compares feedback signal to internal 2.5 V reference; connects to compensation network with COMP.
6 COMP	Error amp output	Drives PWM comparator; voltage monitored for standby activation; not ground-referenced.
7 SS	Soft-start capacitor node	Charged at 20 µA typical; clamps COMP during ramp-up; enables hiccup-mode fault recovery.
8 V_CC	Signal section supply	UVLO turn-on at 14–16 V; internal 25 V Zener clamp; low quiescent current after disable (330 µA).
9 V_C	Power stage supply	Supplies gate driver; pulsed current path; requires local buffer capacitor to PGND.
10 OUT	High-current gate driver output	1 A peak totem-pole; 10 V high-level at 200 mA; 1.0 V low-level at 250 mA; 100 ns rise/fall with 1 nF load.
11 PGND	Power ground return	Separate from SGND to isolate high di/dt paths; connects to MOSFET source and bulk capacitor negative.
12 SGND	Signal ground reference	Reference for V_REF, V_FB, COMP, SS, DIS; must be star-connected near IC for noise immunity.
13 ISEN	Current sense input	Monitors primary current via sense resistor; 1.2 V fault threshold; 100 ns internal blanking window.
14 DIS	Disable input	Active-high; >2.5 V disables PWM; must not float; ties to SGND if unused.
15 DC-LIM	Duty cycle limiter select	Connected to V_REF → 50% max duty; floating or grounded → 100% max duty.
16 ST-BY	Standby mode control	Resistor to RCT sets standby threshold; connected to V_REF or floating if unused.

Key Features

Feature	Design Value
Programmable Standby Mode	Automatically reduces oscillator frequency under light load by monitoring V_COMP, cutting switching losses without sacrificing heavy-load performance.
Integrated Soft-Start & Hiccup Recovery	Linear SS ramp prevents inrush; overcurrent faults trigger controlled restart cycles (Thic ≈ 4.5·(1/ISSC + 1/ISSD)·Css), avoiding latch-off.
IN/OUT Synchronization	Configurable master/slave operation enables multi-controller phase alignment or external clock injection without additional logic.
100 ns Leading-Edge Blanking	Hardware-based noise rejection on ISEN pin eliminates false triggering during MOSFET turn-on spikes.
Dual-Threshold UVLO	4.5 V hysteresis (L5991) ensures clean start-up and shutdown across wide input ranges, reducing need for external supervision.
Separate Power & Signal Grounds	PGND/SGND isolation maintains signal integrity in high-noise flyback topologies, improving EMI and loop stability.

Applications

AC-DC Wall Adapters

Industrial Auxiliary Supplies

Use Scenario: 5–24 W universal-input (85–265 VAC) isolated adapter for IoT sensors and PLC modules.

IC Role / Device Role / Timing Role: Primary-side PWM controller regulating output via optocoupler feedback; manages soft-start, overvoltage shutdown, and standby frequency scaling.

Use Value: Achieves <0.3 W no-load consumption via standby mode and <120 µA start-up current, meeting Level VI efficiency requirements.

Use Scenario: 15 W auxiliary supply powering microcontrollers and I/O drivers in motor control cabinets.

IC Role / Device Role / Timing Role: Flyback controller with synchronized multi-rail operation; uses SYNC pin to align switching with main converter for reduced EMI.

Use Value: 1 A gate drive eliminates external buffer, while 1 MHz capability allows smaller magnetics and tighter transient response.

Medical Isolated DC-DC Converters

Smart Home Power Modules

Use Scenario: 12 W reinforced-isolation DC-DC module for patient-monitoring front-end analog circuitry.

IC Role / Device Role / Timing Role: Primary controller implementing precise current-mode regulation with hiccup-mode fault containment for safety-critical operation.

Use Value: Internal 1.2 V current-sense threshold and 100 ns blanking ensure reliable short-circuit detection without false trips from layout noise.

Use Scenario: Compact 10 W USB-C PD companion supply in smart lighting hubs and voice assistants.

IC Role / Device Role / Timing Role: Controller managing dynamic load steps and burst-mode transitions; uses ST-BY pin to reduce audible noise at light loads.

Use Value: Programmable 50%/100% duty cycle limits prevent transformer saturation during startup and overload conditions.

Equivalent & Alternatives

The following parts are listed as comparable options for similar primary-side PWM controller applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
UC3844BD1R2G	Fixed 250 kHz frequency; no standby mode; 200 mA output drive; no synchronization or soft-start discharge.	Suitable for cost-sensitive fixed-frequency designs where light-load efficiency is secondary.	Select when board space and BOM count are prioritized over adaptive frequency control and hiccup recovery.
ICE2A265	Integrated 650 V MOSFET; fixed 65 kHz; no programmable duty cycle or external sync; lower start-up current (60 µA).	Targeted at low-power integrated SMPS where primary-side integration reduces component count.	Choose for ultra-compact designs needing monolithic solution, but accept trade-offs in flexibility and peak drive strength.

Compared with UC3844BD1R2G and ICE2A265, the L5991D provides superior adaptability through its 1 MHz programmability, 1 A gate drive, and autonomous standby frequency scaling-enabling higher power density and lower no-load losses in variable-load applications.

Availability

L5991D is available at Aetrix Electronics and suitable for AC-DC adapters, industrial auxiliary supplies, and medical isolated DC-DC converters requiring stable component supply, long-term lifecycle support, and consistent parametric performance across production batches.

Supply support for L5991D 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, power, and microcontroller solutions for industrial, automotive, and consumer markets.

The L5991D belongs to ST's MultiPower BCD technology family of high-voltage controllers, engineered specifically for high-efficiency, low-EMI offline flyback and forward converters with integrated protection and adaptive control.

FAQ

What is the minimum recommended value for the soft-start capacitor (Css) on pin 7?

The soft-start capacitor Css should be selected to achieve a desired soft-start time T_ss ≈ 3·R_sense·I_Qpk/(I_SSC·Css). With typical I_SSC = 20 µA and R_sense = 0.2 Ω driving 2 A peak current, a 100 nF capacitor yields ~15 ms ramp. Values below 47 nF risk insufficient ramp duration, while >1 µF may delay system readiness unnecessarily.

How does the ST-BY pin function when used for light-load frequency reduction?

The ST-BY pin connects to RCT via an external resistor to set the threshold at which V_COMP crossing triggers standby mode. When V_COMP falls below 2.5 V, the oscillator switches from f_osc to f_SB; rising above 4.0 V restores full frequency. This occurs automatically without software or external logic, reducing switching losses by up to 60% at light loads.

Can the L5991D synchronize to an external clock while operating in standby mode?

No-the synchronization function is disabled during standby mode. The SYNC pin operates only during normal-frequency operation (f_osc). When the controller enters standby (f_SB), the internal oscillator runs independently, and external sync pulses are ignored until V_COMP rises above VT2 and full-frequency operation resumes.

What is the maximum allowable voltage on the V_C pin (pin 9), and how should it be decoupled?

V_C has no absolute maximum rating beyond the IC's absolute maximum V_CC = 25 V, but it must be supplied from a low-impedance source capable of delivering pulsed current. A minimum 10 µF ceramic+electrolytic buffer capacitor (rated ≥25 V) must be placed between V_C and PGND, located within 5 mm of the IC, to sustain gate drive transients and prevent supply droop.

L5991D Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

Package/Case:: 16-SOIC (0.154", 3.90mm Width)

Packaging:: Tube

Product Status:: Active

Output Isolation:: Isolated

Internal Switch(s):: No

Voltage - Breakdown:: -

Topology:: Boost, Flyback

Voltage - Start Up:: 15 V

Voltage - Supply (Vcc/Vdd):: 10V ~ 20V

Duty Cycle:: 93%

Frequency - Switching:: Up to 1MHz

Power (Watts):: -

Fault Protection:: Current Limiting, Over Voltage

Control Features:: Frequency Control, Soft Start, Sync

Operating Temperature:: -40°C ~ 150°C (TJ)

Grade:: -

Qualification:: -

Supplier Device Package:: 16-SO

Mounting Type:: Surface Mount

L5991D FAQ

1.How can I place an order for L5991D through Aetrix?

Please submit a Request for Quotation (RFQ) for L5991D on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for L5991D reliable?

The price and inventory of L5991D are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for L5991D is usually 5 days.

3.What payment methods are accepted for L5991D?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for L5991D transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for L5991D?

L5991D orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your L5991D order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for L5991D?

For technical support, including L5991D datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your L5991D requirements.

6.How does Aetrix verify that L5991D is sourced from the original manufacturer or authorized distributors?

All L5991D products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that L5991D meets industry standards.

7.What is the process for return or replacement of L5991D?

All L5991D units undergo pre-shipment inspection (PSI). If there is an issue with L5991D, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The L5991D part is unused and in its original packaging.

Return procedure for L5991D:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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