LTC4121/LTC4121-4.2 - 40V 400mA Synchronous Step-Down Battery Charger


  • Wide Input Voltage Range: 4.4V to 40V
  • Temperature Compensated Input Voltage Regulation for Maximum Power Point Tracking (MPPT)
  • Adjustable Float Voltage 3.5V to 18V (LTC4121)
  • Fixed 4.2V Float Voltage Option (LTC4121-4.2)
  • High Efficiency: Up to 95%
  • 50mA to 400mA Programmable Charge Current
  • ±1% Feedback Voltage Accuracy
  • Programmable 5% Accurate Charge Current
  • Thermally Enhanced, Low Profile (0.75mm) 16-Lead (3mm × 3mm) QFN Package

Typical Application

LTC4121 Typical Application
LTC4121 Typical Application


The LTC®4121 is a 400mA constant-current/constantvoltage (CC/CV) synchronous step-down battery charger. In addition to CC/CV operation, the LTC4121 regulates its input voltage to a programmable percentage of the input open-circuit voltage. This technique maintains maximum power transfer with high impedance input sources such as solar panels.

An external resistor programs the charge current up to 400mA. The LTC4121-4.2 is suitable for charging Li-Ion/ Polymer batteries, while the programmable float voltage of the LTC4121 is suitable for several battery chemistries.

The LTC4121 and LTC4121-4.2 include an accurate RUN pin threshold, low voltage battery preconditioning and bad battery fault detection, timer termination, auto-recharge, and NTC temperature qualified charging. The FAULT pin provides an indication of bad battery or temperature faults.

Once charging is terminated, the LTC4121 signals end-ofcharge via the CHRG pin, and enters a low current SLEEP mode. An auto-restart feature starts a new charging cycle if the battery voltage drops by 2.2%.


CAD Symbols and Footprints: The downloadable Zip file below contains the schematic symbol and PCB footprints.

For complete and up to date package information and drawings, please refer to our packaging page

Part Number Package Code Temp Package
LTC4121EUD-4.2#PBF 3x3 QFN-16 UD E 05-08-1691 Yes
LTC4121EUD-4.2#TRPBF 3x3 QFN-16 UD E 05-08-1691 Yes
LTC4121EUD#PBF 3x3 QFN-16 UD E 05-08-1691 Yes
LTC4121EUD#TRPBF 3x3 QFN-16 UD E 05-08-1691 Yes
LTC4121IUD-4.2#PBF 3x3 QFN-16 UD I 05-08-1691 Yes
LTC4121IUD-4.2#TRPBF 3x3 QFN-16 UD I 05-08-1691 Yes
LTC4121IUD#PBF 3x3 QFN-16 UD I 05-08-1691 Yes
LTC4121IUD#TRPBF 3x3 QFN-16 UD I 05-08-1691 Yes

LTC4121 Package Drawing
LTC4121 Package Drawing

Order Info

  • Part numbers ending in PBF are lead free. Solder plated terminal finish (SnPb) versions are non-standard and special terms and conditions and pricing applies if available. Please contact LTC marketing for information.
  • Part numbers containing TR or TRM are shipped in tape and reel or 500 unit mini tape and reel, respectively
  • Please refer to our general ordering information or the product datasheet for more details

Package Variations and Pricing

Part Number Package Temp Price
LTC4121EUD-4.2#PBF 3x3 QFN-16 E $3.71 $2.60 Yes
LTC4121EUD-4.2#TRPBF 3x3 QFN-16 E $2.66 Yes
LTC4121EUD#PBF 3x3 QFN-16 E $3.71 $2.60 Yes
LTC4121EUD#TRPBF 3x3 QFN-16 E $2.66 Yes
LTC4121IUD-4.2#PBF 3x3 QFN-16 I $4.29 $3.00 Yes
LTC4121IUD-4.2#TRPBF 3x3 QFN-16 I $3.06 Yes
LTC4121IUD#PBF 3x3 QFN-16 I $4.29 $3.00 Yes
LTC4121IUD#TRPBF 3x3 QFN-16 I $3.06 Yes
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* The USA list pricing shown is for BUDGETARY USE ONLY, shown in United States dollars (FOB USA per unit for the stated volume), and is subject to change. International prices may differ due to local duties, taxes, fees and exchange rates. For volume-specific price or delivery quotes, please contact your local Linear Technology sales office or authorized distributor.

Demo Boards

Linear Technology offers many demo boards free of charge to qualified customers. Contact your local sales office or distributor to inquire about a demo board. Certain demo boards are also available for sale via credit card on this website. Demo boards are for evaluation purposes only. It remains the customer’s responsibility to verify proper and reliable operation in the actual end application.

Part Number Description Price Documentation
DC1977A-A LTC4121EUD-4.2 Demo Board | 400mA Sync Buck Li Ion Battery Charger, VIN = 4.4V to 40V $125.00
DC1977A-B LTC4121EUD Demo Board | 400mA Synchronous Buck Battery Charger, VIN = 4.4V to 40V, VBAT = Adjustable $125.00
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