Datasheet LTC3121 (Analog Devices) - 10

LTC3121
operaTion Internal Current Limit
shutdown, and draw no current from the input source. It The current limit comparator shuts off the N-channel also allows for inrush current limiting at turn-on, minimiz- MOSFET switch once its threshold is reached. Peak switch ing surge currents seen by the input supply. Note that to current is limited to 1.8A, independent of input or output obtain the advantages of output disconnect, there must voltage, except when V not be an external Schottky diode connected between SW OUT is below 1.5V, resulting in the current limit being approximately half of the nominal peak. and VOUT. The output disconnect feature also allows VOUT to be pulled high, without reverse current being backfed Lossless current sensing converts the peak current sig- into the power source connected to VIN. nal of the N-channel MOSFET switch into a voltage that is summed with the internal slope compensation. The
Shutdown
summed signal is compared to the error amplifier output The boost converter is disabled by pulling SD below 0.25V to provide a peak current control command for the PWM. and enabled by pulling SD above 1.6V. Note that SD can
Zero Current Comparator
be driven above VIN or VOUT, as long as it is limited to less than the absolute maximum rating. The zero current comparator monitors the inductor current being delivered to the output and shuts off the synchro-
Thermal Shutdown
nous rectifier when this current reduces to approximately If the die temperature exceeds 170°C typical, the LTC3121 50mA. This prevents the inductor current from reversing will go into thermal shutdown (TSD). All switches will be in polarity, improving efficiency at light loads. turned off until the die temperature drops by approximately
Oscillator
7°C, when the device re-initiates a soft-start and switching can resume. The internal oscil ator is programmed to the desired switch- ing frequency with an external resistor from the RT pin to
Boost Anti-Ringing Control
GND according to the following formula: When VOUT ≥ VIN + 2V, the anti-ringing control connects ⎛ ⎞ a resistor across the inductor to damp high frequency ƒ 57.6 ⎜ ⎟ OSC (MHz) = ringing on the SW pin during discontinuous current mode ⎝RT(kΩ)⎠ operation when the inductor current has dropped to near The oscillator also can be synchronized to an external zero. Although the ringing of the resonant circuit formed frequency by applying a pulse train to the PWM/SYNC pin. by L and CSW (capacitance on SW pin) is low energy, it An external resistor must be connected between RT and can cause EMI radiation. GND to program the oscillator to a frequency approximately 25% below that of the externally applied pulse train used
VCC Regulator
for synchronization. RT is selected in this case according An internal low dropout regulator generates the 4.25V to this formula: (nominal) VCC rail from VIN or VOUT, depending upon ⎛ ⎞ operating conditions. VCC is supplied from VIN when VIN R 73.2 T(kΩ) = ⎜ ⎟ is greater than 3.5V, otherwise the greater of VIN and VOUT ⎝ ƒSYNC(MHz)⎠ is used. The VCC rail powers the internal control circuitry and power MOSFET gate drivers of the LTC3121. The VCC
Output Disconnect
regulator is disabled in shutdown to reduce quiescent The LTC3121’s output disconnect feature eliminates body current and is enabled by forcing the SD pin above its diode conduction of the internal P-channel MOSFET threshold. A 4.7µF or larger capacitor must be connected rectifier. This allows for V between V OUT to discharge to 0V during CC and SGND. 3121fa 10 For more information www.linear.com/LTC3121 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Low Voltage Operation Low Noise Fixed Frequency Operation Burst Mode Operation Applications Information PCB Layout Guidelines Schottky Diode Component Selection Operating Frequency Selection Typical Applications Package Description Revision History Typical Application Related Parts