Datasheet LTC7138 (Analog Devices) - 10

LTC7138
applicaTions inForMaTion
The basic LTC7138 application circuit is shown on the front 800 page of this data sheet. External component selection is 700 determined by the maximum load current requirement and 600 MAXIMUM PEAK begins with the selection of the peak current programming INDUCTOR CURRENT resistor, R 500 ISET. The inductor value L can then be determined, followed by capacitors C 400 IN and COUT. CURRENT (mA) 300
Maximum Output Current
MAXIMUM 200 LOAD The maximum average output current is determined by CURRENT 100 the peak current trip threshold and the valley current trip 0 threshold. With the I 0 25 50 75 100 125 150 175 200 225 250 SET pin open, the peak current com- R parator has a minimum threshold of 540mA. The valley ISET (kΩ) 7138 F02 current comparator has a minimum threshold of 50%
Figure 2. RISET Selection
of the peak current, or 270mA. At maximum load, the inductor current ramps between the peak and valley cur- where 100mA < IOUT(MAX) < 405mA. This equation gives rent thresholds, which results in a maximum load current the maximum load current supplied using the minimum that is the average of the two, or 405mA. For applications peak and valley current. For inductor selection, the maxi- that demand less current, the peak current threshold can mum peak current can then be approximated for a given be reduced to as low as 140mA, which provides 100mA RISET resistor value as: average output current. This lower peak current allows the 3.3mA efficiency and component selection to be optimized for IPEAK(MAX) ≈RISET • +30mA 1kΩ lower current applications. For applications that require more than 400mA, multiple LTC7138s can be connected The peak current is internally limited to be within the range in parallel using the FBO pin. See the Higher Current Ap- of 140mA to 540mA. Shorting the ISET pin to ground pro- plications section for more information. grams the current limit to 140mA (100mA average output The peak current threshold is linearly proportional to the current), and leaving it floating sets the current limit to the voltage on the I maximum value of 540mA (405mA average output current). SET pin, with 280mV and 1V corresponding to 140mA and 540mA peak current, respectively. The valley The internal 5µA current source is reduced to 1µA in sleep current threshold correspondingly changes with the volt- mode to maximize efficiency and to facilitate a trade-off age on the I between efficiency and light load output voltage ripple, as SET pin to remain at 50% of the programmed peak current. This pin may be driven by an external volt- described in the Optimizing Output Voltage Ripple section. age source to modulate the peak current, which may be
Inductor Selection
beneficial in some applications. Usually, the peak current is programmed with an appropriately chosen resistor (R For the LTC7138, which has relatively low output current ISET) between the I and very high input voltage, switching losses typically SET pin and ground. The voltage generated on the I dominate the power loss equation. For this architecture, SET pin by RISET and the internal 5µA current source sets the peak current. The value of resistor to achieve a higher inductor values lower the switching frequency maximum average output current can be computed by which decreases switching loss at the expense of higher using Figure 2 or the following equation: DC resistance and lower saturation current. Therefore choosing the largest inductor value that satisfies both 1kΩ R ISET =IOUT(MAX) • 2mA 7138f 10 For more information www.linear.com/LTC7138 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