Datasheet LTC1174, LTC1174-3.3, LTC1174-5 (Analog Devices) - 8

LTC1174 LTC1174-3.3/LTC1174-5
U U W U APPLICATIO S I FOR ATIO Inductor Core Selection
a premium larger gauge wire can be used to reduce the wire With the value of L selected, the type of inductor must be resistance. This also prevents excessive heat dissipation. chosen. Basically there are two kinds of losses in an
C
inductor, core and copper
IN
In continuous mode the source current of the P-channel Core losses are dependent on the peak-to-peak ripple MOSFET is a square wave of duty cycle V current and the core material. However it is independent of OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the physical size of the core. By increasing the inductance the maximum RMS current must be used. The C the inductor’s peak-to-peak ripple current will decrease, IN RMS current is given by: therefore reducing core loss. Utilizing low core loss mate- rial, such as molypermalloy or Kool Mµ will allow users to [ ( )]1/2 concentrate on reducing copper loss and preventing satu- I V V − V OUT OUT IN OUT I ≈ (A ) ration. Figure 1 shows the effect of different core material on RMS V RMS IN the efficiency of the LTC1174. The CTX core is Kool Mµ and the CTXP core is powdered iron (material 52). This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst case is commonly used for design Although higher inductance reduces core loss, it increases because even significant deviations do not offer much relief. copper loss as it requires more windings. When space is not Note that ripple current directly affects capacitor’s lifetime. DO NOT UNDERSPECIFY THIS COMPONENT. An additional 100 0.1µF ceramic capacitor is also required on VIN for high frequency decoupling. CTX100-4 90
C
CTX100-4P
OUT
80 To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The 70 EFFICIENCY (%) worst case RMS ripple current in the output capacitor is given by: 60 VIN = 5V VOUT = 3.3V IPGM = VIN I 50 I PEAK ≈ (A ) 1 10 100 500 RMS RMS 2 LOAD CURRENT (mA) = 170mA or 300mA 100 Although the output voltage ripple is determined by the CTX50-4 90 hysteresis of the voltage comparator, ESR of the output capacitor is also a concern. Too high of an ESR will create CTX50-4P 80 a higher ripple output voltage and at the same time cause the LTC1174 to sleep less often. This will affect the efficiency of 70 EFFICIENCY (%) the LTC1174. For a given technology, ESR is a direct function of the volume of the capacitor. Several small-sized 60 VIN = 5V VOUT = 3.3V capacitors can also be paralleled to obtain the same ESR as IPGM = VIN one large can. Manufacturers such as Nichicon, Chemicon 50 1 10 100 500 and Sprague should be considered for high performance LOAD CURRENT (mA) 1174 F01 capacitors. The OS-CON semiconductor dielectric capaci- tor available from Sanyo has the lowest ESR for its size, at
Figure 1. Efficiency Using Different Types of Inductor Core Material
a higher price. 1174fe 8