September 27, 2011
LM21305
5A Adjustable Frequency Synchronous Buck Regulator
General Description
Key Specifications
The LM21305 is a full-featured 5A synchronous buck POL
regulator optimized for solution size, flexibilty, and high con-
version efficiency. High power density LM21305 designs are
achieved via monolithic integration of the high-side and low-
side power MOSFETs, high switching frequency, current-
mode control, and optimized thermal design. The efficiency
of the LM21305 is elevated at light loads with diode emulation
mode operation and at heavy loads by optimal design of the
MOSFET gate drivers to minimize switch dead-times and
body-diode conduction losses.
Single-rail input voltage range from 3V to 18V
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0.598V feedback voltage reference
300 kHz to 1.5 MHz switching frequency range
LLP-28 package (5 x 5 x 0.8 mm, 0.5 mm pitch)
Features
High efficiency switcher core with integrated low RDSon
power MOSFETs
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Resistor programmable switching frequency with
frequency synchronization
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The LM21305 accepts a wide input voltage range of 3V to
18V, facilitating interface to all intermediate bus voltages, in-
cluding 3.3V, 5V and 12V rails. An output voltage as low as
0.598V is supported with excellent setpoint accuracy and low
ripple and RMS noise. The LM21305 offers flexible system
configuration with programmable switching frequency from
300 kHz to 1.5 MHz using one resistor or via external clock
synchronization. On-chip bias supply sub-regulators alleviate
the need for external bias power and simplify PCB layout. The
device also offers internal soft-start to limit inrush current, pre-
biased and monotonic startup capability, cycle-by-cycle cur-
rent limiting, and thermal shutdown. Peak current-mode
control with a high-gain error amplifier maintains stability
throughout the entire input voltage and load current ranges
and enables excellent line and load transient response.
Internal soft-start with monotonic and pre-biased startup
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Low shutdown quiescent current
Precision enable with hysteresis
PGOOD indicator function
Input under-voltage lock-out (UVLO)
Output over-voltage protection (OVP)
High-bandwidth load transient response with peak
current-mode control
Cycle-by-cycle current limiting
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Thermal shutdown
Applications
Offered in a thermally enhanced LLP-28 package, the
LM21305 features internal output over-voltage and over-cur-
rent protection circuits for increased system reliability. A pre-
cision enable pin and integrated input UVLO allow the turn-
on of the device to be tightly controlled and sequenced. An
integrated open-drain power good indicator provides power
rail sequencing capability and fault indication.
POL regulation from 3.3V, 5V, and 12V supply rails
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High efficiency supply for DSPs, FPGAs, ASICs and
processors
Broadband, networking and optical communications
infrastructure
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Typical Application Circuit
30111101
© 2011 National Semiconductor Corporation
301111
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