SY21243B3RHC - High Efficiency Fast Response, 8A, 28V Input Synchronous Step Down Regulator with 100mA 3.3V LDO


  • Low RDS(ON) for Internal Switches (top/bottom): 20/10 mΩ
  • Wide Input Voltage Range: 4~24V
  • Integrated Bypass Switch: 1.5Ω
  • Instant PWM Architecture to Achieve Fast Transient Responses
  • Internal Soft-start Limits the Inrush Current
  • Pseudo-constant Frequency: 600kHz
  • Fixed 3.338V Output Voltage
  • 8A Output Current Capability
  • 100mA LDO Current Capability
  • ±1% Internal Reference Voltage
  • PFM/USM Selectable Light Load Operation Mode
  • Power Good Indicator
  • Output Discharge Function
  • Cycle-by-cycle Valley and Peak Current Limit Protection
  • Latch-off Mode Output Under Voltage Protection for Buck
  • Latch-off Mode Output Over Voltage Protection for Buck
  • Latch-off Mode Over Temperature Protection for Buck
  • Auto-recovery Mode Output Under Voltage Protection for LDO
  • Auto-recovery Mode Over Temperature Protection for LDO
  • Input Under Voltage Lock-out(UVLO)
  • RoHS Compliant and Halogen Free
  • Compact package: QFN2.5×2.5-16

Typical Application


The SY21243B3 develops a high efficiency synchronous step-down DC/DC regulator capable of delivering 8A current. The device integrates main switch and synchronous switch with very low RDS(ON) to minimize the conduction loss. In addition, it operates at pseudo-constant frequency of 600kHz under heavy load conditions to minimize the size of inductor and capacitor.  SY21243B3 also provides a fixed 3.3V LDO with 100mA current capability, which can be used to power the external peripheries, such as the keyboard controller in notebook. The 3.3V LDO can switch over to Buck regulator output to save power loss.

Silergy’s proprietary Instant-PWM™ fast-response, constant-on-time (COT) PWM control method supports high input/output voltage ratios (low duty cycles), and fast transient response while maintaining a near constant operating frequency over line, load and output voltage ranges. This control method provides stable operation without complex compensation and even with low ESR ceramic capacitors.

The SY21243B3 operates over a wide input voltage range from 4V to 24V. Cycle-by-cycle current limit, input under voltage lock-out, internal soft-start, output under voltage protection, over voltage protection and thermal shutdown provide safe operation in all operating conditions.


  • LCD-TV/Net-TV/3DTV
  • Set Top Box
  • Notebook
  • High Power AP