Datasheet LTC3639 (Analog Devices) - 10

LTC3639
APPLICATIONS INFORMATION
The basic LTC3639 application circuit is shown on the The internal 5μA current source is reduced to 1μA in sleep front page of this data sheet. External component selec- mode to maximize efficiency and to facilitate a tradeoff tion is determined by the maximum load current require- between efficiency and light load output voltage ripple, as ment and begins with the selection of the peak current described in the Optimizing Output Voltage Ripple section. programming resistor, RISET. The inductor value L can The peak current is internally limited to be within the range then be determined, followed by capacitors CIN and COUT. of 20mA to 200mA. Shorting the ISET pin to ground pro-
Peak Current Resistor Selection
grams the current limit to 20mA, and leaving it floating sets the current limit to the maximum value of 200mA. When The peak current comparator has a maximum current selecting this resistor value, be aware that the maximum limit of at least 200mA, which guarantees a maximum average output current for this architecture is limited to average current of 100mA. For applications that demand half of the peak current. Therefore, be sure to select a value less current, the peak current threshold can be reduced that sets the peak current with enough margin to provide to as little as 17mA. This lower peak current allows the adequate load current under all conditions. Selecting the efficiency and component selection to be optimized for peak current to be 2.2 times greater than the maximum lower current applications. load current is a good starting point for most applications. The peak current threshold is linearly proportional to the voltage on the I
Inductor Selection
SET pin, with 100mV and 1V corresponding to 20mA and 200mA peak current respectively. This pin The inductor, input voltage, output voltage, and peak current may be driven by an external voltage source to modulate determine the switching frequency during a burst cycle of the peak current, which may be beneficial in some applica- the LTC3639. For a given input voltage, output voltage, tions. Usually, the peak current is programmed with an and peak current, the inductor value sets the switching appropriately chosen resistor (RISET) between the ISET pin frequency during a burst cycle when the output is in regu- and ground. The voltage generated on the ISET pin by RISET lation. Generally, switching at a frequency between 50kHz and the internal 5µA current source sets the peak current. and 200kHz yields high efficiency, and 100kHz is a good The value of resistor for a particular peak current can be first choice for many applications. The inductor value can computed by using Figure 2 or the following equation: be determined by the following equation: RISET = IPEAK • 106  V   V  L = OUT OUT where 20mA < IPEAK < 200mA.  f •I  • 1–  V  PEAK IN 250 The variation in switching frequency during a burst cycle with input voltage and inductance is shown in Figure 3. For 200 lower values of I TYPICAL PEAK PEAK, multiply the frequency in Figure 3 INDUCTOR by 230mA/I 150 PEAK. CURRENT An additional constraint on the inductor value is the 100 LTC3639’s 150ns minimum on-time of the switches. CURRENT (mA) Therefore, in order to keep the current in the inductor 50 MAXIMUM LOAD CURRENT 0 0 25 50 75 100 125 150 175 200 RISET (kΩ) 3639 F02
Figure 2. RISET Selection
3639fc 10 For more information www.linear.com/LTC3639 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 Revision History Typical Application Related Parts