Datasheet LTC3121 (Analog Devices) - 9

LTC3121
operaTion
The LTC3121 is an adjustable frequency, 100kHz to 3MHz zero to its final programmed value. This limits the inrush synchronous boost converter housed in a 12-lead 4mm × current drawn from the input source. As a result, the du- 3mm DFN. The LTC3121 offers the unique ability to start- ration of the soft-start is largely unaffected by the size of up and regulate the output from inputs as low as 1.8V and the output capacitor or the output regulation voltage. The continue to operate from inputs as low as 0.5V. Output closed loop nature of the soft-start allows the converter voltages can be programmed between 2.2V and 15V. The to respond to load transients that might occur during device also features fixed frequency, current mode PWM the soft-start interval. The soft-start period is reset by a control for exceptional line and load regulation. The cur- shutdown command on SD, a UVLO event on VCC (VCC < rent mode architecture with adaptive slope compensation 1.6V), an overvoltage event on VOUT (VOUT ≥ 16.2V), or provides excellent transient load response and requires an overtemperature event (thermal shutdown is invoked minimal output filtering. An internal 10ms closed loop when the die temperature exceeds 170°C). Upon removal soft-start simplifies the design process while minimizing of these fault conditions, the LTC3121 will soft-start the the number of external components. output voltage. With its low RDS(ON) and low gate charge internal N-channel
Error Amplifier
MOSFET switch and P-channel MOSFET synchronous rectifier, the LTC3121 achieves high efficiency over a wide The non-inverting input of the transconductance error range of load current. High efficiency is achieved at light amplifier is internally connected to the 1.202V reference loads when Burst Mode operation is commanded. Operation and the inverting input is connected to FB. An external can be best understood by referring to the Block Diagram. resistive voltage divider from VOUT to ground programs the output voltage from 2.2V to 15V via FB as shown in
LOW VOLTAGE OPERATION
Figure 1. ⎛ R1 The LTC3121 is designed to allow start-up from input V ⎞ OUT = 1.202V⎜1+ ⎟ voltages as low as 1.8V. When V ⎝ R2⎠ OUT exceeds 2.2V, the LTC3121 continues to regulate its output, even when VIN Selecting an R2 value of 121kΩ to have approximately falls to as low as 0.5V. The limiting factors for the applica- 10µA of bias current in the V tion become the availability of the input source to supply OUT resistor divider yields the formula: sufficient power to the output at the low voltages, and the maximum duty cycle. Note that at low input voltages, R1 = 100.67•(VOUT – 1.202V) small voltage drops due to series resistance become where R1 is in kΩ. critical and greatly limit the power delivery capability of the converter. This feature extends operating times by Power converter control loop compensation is set by a maximizing the amount of energy that can be extracted simple RC network between VC and ground. from the input source. VOUT LTC3121
LOW NOISE FIXED FREQUENCY OPERATION
R1 – FB
Soft-Start
+ 1.202V R2 The LTC3121 contains internal circuitry to provide closed- loop soft-start operation. The soft-start utilizes a linearly increasing ramp of the error amplifier reference voltage 3121 F01 from zero to its nominal value of 1.202V in approximately 10ms, with the internal control loop driving V
Figure 1. Programming the Output Voltage
OUT from 3121fa For more information www.linear.com/LTC3121 9 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