Datasheet LTC3122 (Analog Devices) - 9

LTC3122
operaTion
The LTC3122 is an adjustable frequency, 100kHz to 3MHz zero to its final programmed value. This limits the inrush synchronous boost converter housed in either a 12-lead current drawn from the input source. As a result, the du- 4mm × 3mm DFN or a thermally enhanced MSOP pack- ration of the soft-start is largely unaffected by the size of age. The LTC3122 offers the unique ability to start-up the output capacitor or the output regulation voltage. The and regulate the output from inputs as low as 1.8V and closed loop nature of the soft-start allows the converter continue to operate from inputs as low as 0.5V. Output to respond to load transients that might occur during voltages can be programmed between 2.2V and 15V. The the soft-start interval. The soft-start period is reset by a device also features fixed frequency, current mode PWM shutdown command on SD, a UVLO event on VCC (VCC < control for exceptional line and load regulation. The cur- 1.6V), an overvoltage event on VOUT (VOUT ≥ 16.2V), or rent mode architecture with adaptive slope compensation an overtemperature event (thermal shutdown is invoked provides excellent transient load response and requires when the die temperature exceeds 170°C). Upon removal minimal output filtering. An internal 10ms closed loop of these fault conditions, the LTC3122 will soft-start the soft-start simplifies the design process while minimizing output voltage. the number of external components.
Error Amplifier
With its low RDS(ON) and low gate charge internal N-channel MOSFET switch and P-channel MOSFET synchronous The non-inverting input of the transconductance error rectifier, the LTC3122 achieves high efficiency over a wide amplifier is internally connected to the 1.202V reference range of load current. High efficiency is achieved at light and the inverting input is connected to FB. An external loads when Burst Mode operation is commanded. Operation resistive voltage divider from VOUT to ground programs can be best understood by referring to the Block Diagram. the output voltage from 2.2V to 15V via FB as shown in Figure 1.
LOW VOLTAGE OPERATION
V ⎛ R1⎞ OUT = 1.202V⎜1+ ⎟ The LTC3122 is designed to allow start-up from input ⎝ R2⎠ voltages as low as 1.8V. When VOUT exceeds 2.2V, the Selecting an R2 value of 121kΩ to have approximately LTC3122 continues to regulate its output, even when VIN 10µA of bias current in the V falls to as low as 0.5V. The limiting factors for the applica- OUT resistor divider yields the formula: tion become the availability of the input source to supply sufficient power to the output at the low voltages, and R1 = 100.67•(VOUT – 1.202V) the maximum duty cycle. Note that at low input voltages, where R1 is in kΩ. small voltage drops due to series resistance become critical and greatly limit the power delivery capability of Power converter control loop compensation is set by a the converter. This feature extends operating times by simple RC network between VC and ground. maximizing the amount of energy that can be extracted from the input source. VOUT LTC3122 R1
LOW NOISE FIXED FREQUENCY OPERATION
– FB +
Soft-Start
1.202V R2 The LTC3122 contains internal circuitry to provide closed- loop soft-start operation. The soft-start utilizes a linearly 3122 F01 increasing ramp of the error amplifier reference voltage from zero to its nominal value of 1.202V in approximately
Figure 1. Programming the Output Voltage
10ms, with the internal control loop driving VOUT from 3122fa For more information www.linear.com/LTC3122 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 Applications Information Typical Applications Package Description Typical Application Related Parts