Datasheet LTC1624 (Analog Devices) - 9

LTC1624
U U W U APPLICATIONS INFORMATION
monly used for design because even significant deviations ratings that are ideal for input capacitor applications. do not offer much relief. Note that capacitor manufacturer’s Consult the manufacturer for other specific recommend- ripple current ratings are often based on only 2000 hours ations. of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher
INTVCC Regulator
temperature than required. Several capacitors may also be An internal regulator produces the 5V supply that powers paralleled to meet size or height requirements in the the drivers and internal circuitry within the LTC1624. design. Always consult the manufacturer if there is any Good VIN bypassing is necessary to supply the high question. transient currents required by the MOSFET gate drivers. The selection of COUT is driven by the required effective High input voltage applications in which large MOSFETs series resistance (ESR). Typically, once the ESR require- are being driven at high frequencies may cause the maxi- ment is satisfied the capacitance is adequate for filtering. mum junction temperature rating for the LTC1624 to be The output ripple (∆VOUT) is determined by: exceeded. The supply current is dominated by the gate charge supply current as discussed in the Efficiency  1  ∆VOUT ≈ ∆I ESR + Considerations section. The junction temperature can be L 4fCOUT estimated by using the equations given in Note 1 of the Electrical Characteristics table. For example, the LTC1624 where f = operating frequency, COUT = output capacitance is limited to less than 17mA from a 30V supply: and ∆IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since T ∆I J = 70°C + (17mA)(30V)(110°C/ W) = 126°C L increases with input voltage. With ∆IL = 0.4IOUT(MAX) the output To prevent maximum junction temperature from being ripple will be less than 100mV at maximum VIN, assuming: exceeded, the input supply current must be checked C operating in continuous mode at maximum V OUT Required ESR < 2RSENSE IN. Manufacturers such as Nichicon, United Chemicon and
Step-Down Converter: Topside MOSFET Driver
SANYO should be considered for high performance
Supply (CB, DB)
through-hole capacitors. The OS-CON semiconductor An external bootstrap capacitor CB connected to the BOOST dielectric capacitor available from SANYO has the lowest pin supplies the gate drive voltage for the topside MOSFET. ESR(size) product of any aluminum electrolytic at a some- Capacitor CB in the functional diagram is charged through what higher price. Once the ESR requirement for COUT has internal diode DB from INTVCC when the SW pin is low. been met, the RMS current rating generally far exceeds When the topside MOSFET is to be turned on, the driver the IRIPPLE(P-P) requirement. places the CB voltage across the gate to source of the In surface mount applications multiple capacitors may MOSFET. This enhances the MOSFET and turns on the have to be paralleled to meet the ESR or RMS current topside switch. The switch node voltage SW rises to VIN handling requirements of the application. Aluminum elec- and the BOOST pin rises to VIN + INTVCC. The value of the trolytic and dry tantalum capacitors are both available in boost capacitor CB needs to be 50 times greater than the surface mount configurations. In the case of tantalum it is total input capacitance of the topside MOSFET. In most critical that the capacitors are surge tested for use in applications 0.1µF is adequate. switching power supplies. An excellent choice is the AVX Significant efficiency gains can be realized by supplying TPS series of surface mount tantalums, available in case topside driver operating voltage from the output, since the heights ranging from 2mm to 4mm. Other capacitor types VIN current resulting from the driver and control currents include SANYO OS-CON, Nichicon WF series and Sprague will be scaled by a factor of (Duty Cycle)/(Efficiency). For 595D series and the new ceramics. Ceramic capacitors are 5V regulators this simply means connecting the BOOST now available in extremely low ESR and high ripple current 9