FAN4855MTC - 500mA Boost Regulator, Adjustable | Fairchild
MPN： FAN4855MTC ✗ 停產FAN4855MTC 概覽
A boost regulator is a type of switch-mode DC-DC converter that steps an input voltage up to a higher regulated output voltage using an inductor, a switching element, and a diode. Within the power management hierarchy, the FAN4855 sits under switching regulators, which belong to voltage regulators and power management ICs. Boost converters are essential in battery-powered systems where the cell voltage sags below the level required by the load, such as converting two NiCd/NiMH cells (approximately 1.8 V to 3.0 V) to a 5 V rail.
Key features include low-power PFM (pulse frequency modulation) control for high efficiency at light loads, an input voltage range extending to 4.5 V, an adjustable output voltage range of approximately 3 V to 5 V, up to 500 mA maximum switch current, and an integrated low battery detect comparator with shutdown mode for power conservation. According to the Fairchild Semiconductor datasheet, the device targets low-voltage DC-DC conversion in 2-cell battery-powered systems.
Technically, the FAN4855 uses asynchronous PFM boost architecture: the internal NPN switch is driven until the current limit or peak threshold is reached, then the inductor energy is released through an external Schottky diode into the output capacitor. PFM operation eliminates the switching-frequency clock of PWM converters, reducing quiescent power and improving efficiency in portable applications.
Typical applications include digital cameras, cell phones, and PDAs, as well as any 2-cell alkaline or NiMH product needing a stable 5 V logic or display rail.
A key design consideration: PFM regulators exhibit variable switching ripple, so output filtering and PCB layout of the inductor and Schottky diode loop require attention for noise-sensitive loads.
This page synthesizes distributor pricing, drop-in alternative data, and practical design notes not found in the manufacturer datasheet.
直接替換替代品，適用於 FAN4855MTC — 相同封裝，腳位對腳位相容。需重新設計 PCB 的不同封裝零件不列入。
FAN4855MTCX
✅ 直接替換📋 參考替代品（不在目錄中）
FAN4855MTCX3
✅ 直接替換📋 參考替代品（不在目錄中）
FAN4855MTCX4
✅ 直接替換📋 參考替代品（不在目錄中）
FAN4855B
✅ 直接替換📋 參考替代品（不在目錄中）
FAN4855MTC 最大額定值與電氣特性
| 拓撲 | Boost (step-up) switching regulator |
| Control Scheme | PFM (pulse frequency modulation) |
| 輸出類型 | Positive, Adjustable |
| 輸出電壓範圍 | 3 V to 5 V |
| 輸出數量 | 1 |
| Maximum Switch Current | 500 mA |
| Special Features | Shutdown, Low Battery Detect |
| 封裝 | 8-TSSOP (0.173 in, 4.40 mm width) |
| 腳位數 | 8 |
| 安裝類型 | Surface Mount |
| Typical Applications | 2-cell battery powered systems (digital cameras, cell phones, PDAs) |
| Efficiency Class | High efficiency (500 mA class) |
| Manufacturer | Fairchild Semiconductor (now onsemi) |
FAN4855MTC 8-tssop (0.173 in, 4.40 mm width) 腳位配置指南
FAN4855MTC（8-tssop (0.173 in, 4.40 mm width) 封裝）的腳位配置。 此功率元件具有閘極、汲極與源極端子。對於非極性封裝，請參閱製造商資料表以取得確切的第 1 腳方向與佈局詳細資訊。常見應用包括電源設計、馬達驅動與負載切換。
無詳細腳位定義資料： FAN4855MTC.
請參閱 資料表 以查看完整腳位配置。
典型應用
FAN4855MTC 適用於 6 應用s: Digital Camera Power Supply, Cell Phone / PDA Boost Rails, Two-Cell Battery Portable Instruments, 5 V Logic Rail from Low-Voltage Bus, Legacy Board Repair and Sustainment, Low Battery Detection Subsystem.
Digital Camera Power Supply
The FAN4855MTC was designed explicitly for digital cameras powered by two alkaline or NiMH cells, per the Fairchild datasheet general description. Two cells deliver 1.8 V to 3.0 V across discharge, while the LCD backlight, MCU, and flash controller typically require a regulated 5 V rail; the FAN4855MTC boosts this with its 3 V to 5 V adjustable output and 500 mA switch current limit. The integrated low battery detect output warns the camera firmware before the cell voltage collapses, enabling an orderly shutdown and preventing image corruption. Its PFM control keeps efficiency high at the light average loads typical of standby and menu operation. Externally, only an inductor, Schottky diode, and a few capacitors are needed, minimizing BOM cost in consumer hardware.
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Cell Phone / PDA Boost Rails
Early cell phones and PDAs used two or three cells and needed a stable higher rail for the display, backlight, and logic. The FAN4855MTC addresses this with its low-power PFM boost architecture and 4.5 V maximum input rating, per the Fairchild datasheet. In this role the converter sits between the battery pack and the 5 V system rail, with the shutdown pin allowing the baseband processor to disable the boost during deep sleep, cutting quiescent drain to preserve standby time. The adjustable feedback divider lets the same PCB support 3.3 V or 5 V product variants. Because PFM switching ripple is broadband and load-dependent, designers should budget adequate output capacitance and place the boost loop away from the RF section to protect receiver sensitivity.
推薦
Two-Cell Battery Portable Instruments
Handheld meters, sensors, and loggers running from two AA or AAA cells benefit from the FAN4855MTC's combination of adjustable output and low battery detect. The feedback network sets any rail from 3 V to 5 V, allowing designers to power 5 V ADCs or 3.3 V microcontrollers directly from declining cell voltage, and the battery-detect comparator can drive an LED or interrupt the MCU when cells approach end of discharge, per the Fairchild datasheet feature set. The 500 mA switch current supports typical measurement front-ends comfortably. With only PFM burst switching at light load, self-consumption stays low enough for months of intermittent-use battery life. Designers should select a low-DCR inductor and a Schottky diode with low forward voltage to preserve efficiency across the full 1.8 V to 3 V input span.
推薦
5 V Logic Rail from Low-Voltage Bus
Any system with a nominal 3 V to 3.3 V rail that also needs 5 V for legacy peripherals, op-amps, or programming interfaces can use the FAN4855MTC as a local boost stage. With input well within the 4.5 V maximum rating and output adjustable to 5 V, it acts as a point-of-load step-up converter, per distributor and datasheet listings. The shutdown pin allows a host microcontroller to gate the 5 V rail off when unused, reducing idle power. In mixed-voltage boards, the low battery detect input can double as an input-undervoltage lockout by tying it to the input divider, protecting downstream devices from brownout. Keep the inductor switching loop compact and the FB divider close to the IC to maintain regulation accuracy.
推薦
Legacy Board Repair and Sustainment
Because the FAN4855MTC is discontinued, its most common contemporary role is repairing cameras, phones, and PDAs from the era in which it was designed. Technicians can substitute the pin-identical FAN4855MTCX, FAN4855MTCX3, or FAN4855MTCX4, which IC-Components lists as the comparable cross-reference products, without any board modification; FindIC confirms the identical TSSOP-8, 8-pin, 3 V to 5 V profile. Aftermarket stock from Rochester Electronics (listed on DigiKey), Wolfchip (26,600 pcs as of Jul 15, 2026), and Heisener ($0.76 unit price, 5,440 pcs, as of 2026-09-14) keeps the part procurable for field service. Verify date codes when ordering broker stock for critical repairs.
推薦
Low Battery Detection Subsystem
Beyond its boost function, the FAN4855MTC integrates a low battery detect comparator, which lets designers implement battery monitoring without a separate supervisory IC, per the Fairchild datasheet title and feature list. The LBD input connects to a resistor divider from the battery; when the cell voltage crosses the programmed threshold, the device flags the system controller. In 2-cell products this supports graceful actions such as dimming the backlight, saving state to memory, or displaying a replace-battery warning before regulation is lost. Combining boost regulation and battery supervision in one 8-TSSOP device reduces BOM count and board area in cost-driven consumer designs, a key reason it was adopted across cameras, cell phones, and PDAs.
推薦
推薦產品摘要
工程參考資料，適用於 FAN4855MTC — 比較、設計指引及合規資訊。
選型指南
與替代品比較
| 參數 | 本產品 | FAN4855MTCX | FAN4855MTCX3 | FAN4855MTCX4 | FAN4855B |
|---|---|---|---|---|---|
| Package | 8-TSSOP (0.173 in, 4.40 mm width) | TSSOP-8 - same | TSSOP-8 - same | TSSOP-8 - same | TSSOP-8 - same |
| Brand | Fairchild Semiconductor (onsemi) | Fairchild Semiconductor (onsemi) | Fairchild Semiconductor (onsemi) | Fairchild Semiconductor (onsemi) | Fairchild Semiconductor (onsemi) |
| Maximum Switch Current | 500 mA | 500 mA | 500 mA | 500 mA | 500 mA (family rating) |
主要差異化特點
- Integrated low battery detect (對比Generic PFM boost controllers)
- PFM control for light-load efficiency (對比PWM boost converters)
- Multiple drop-in packaging variants within the family (對比FAN4855MTCX / FAN4855MTCX3 / FAN4855MTCX4)
設計說明
Keep the boost power loop - input capacitor, inductor, internal switch, Schottky diode, and output capacitor - as compact as possible. PFM burst switching generates broadband ripple, so a short loop minimizes radiated EMI and ground bounce. Place the feedback divider resistors directly adjacent to the FB pin and route the feedback trace away from the inductor and switching node. Use a solid ground plane under the converter area and connect the IC ground pin and capacitor grounds to it with short, wide traces.
The FAN4855 is an asynchronous boost converter: the output diode must be an external Schottky (not a standard-recovery silicon diode), because a slow diode causes large reverse-recovery losses and efficiency collapse at PFM switching rates. Also verify the maximum input voltage of 4.5 V is never exceeded - do not connect three cells or a 5 V USB source directly to the input. Finally, size the inductor saturation current above the 500 mA switch current limit with margin, or regulation will collapse at peak load.
Estimated: at VIN = 2.4 V (2 NiMH cells), VOUT = 5 V and IOUT = 100 mA, input current is roughly VOUT*IOUT/(VIN*efficiency) = 5*100/(2.4*0.8) = 260 mA, well within the 500 mA switch limit, but note input current can approach or exceed the switch limit as battery voltage falls. At end-of-discharge (about 1.8 V), maximum deliverable output current drops substantially; validate load current against the datasheet output-current curves at your minimum expected input voltage before committing to the design.
合規資訊
Compliance status not stated in the provided verified web data; this is an obsolete Fairchild part so consult onsemi product discontinuance notices or Rochester Electronics for RoHS status of available date codes.
