Datasheet LTC3638 (Analog Devices) - 10

LTC3638
APPLICATIONS INFORMATION
The basic LTC3638 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 40mA to 500mA. Shorting the ISET pin to ground pro-
Peak Current Resistor Selection
grams the current limit to 40mA, and leaving it floating sets the current limit to the maximum value of 500mA. When The peak current comparator has a maximum current selecting this resistor value, be aware that the maximum limit of at least 500mA, which guarantees a maximum average output current for this architecture is limited to average current of 250mA. 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 40mA. 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 40mA and 500mA 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 LTC3638. 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 • 400k  V   V  L = OUT OUT where 40mA < IPEAK < 500mA.  f •IPEAK  • 1–  VIN  600 The variation in switching frequency during a burst cycle with input voltage and inductance is shown in Figure 3. For 500 lower values of IPEAK, multiply the frequency in Figure 3 TYPICAL PEAK 400 INDUCTOR by 575mA/IPEAK. CURRENT 300 An additional constraint on the inductor value is the LTC3638’s 150ns minimum on-time of the switch. CURRENT (mA) 200 Therefore, in order to keep the current in the inductor MAXIMUM well-controlled, the inductor value must be chosen so that 100 LOAD CURRENT 0 0 25 50 75 100 125 150 175 200 RISET (kΩ) 3638 F02
Figure 2. RISET Selection
3638fa 10 For more information www.linear.com/LTC3638 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