Datasheet LTC3806 (Analog Devices) - 9

LTC3806
U U W U APPLICATIO S I FOR ATIO INTVCC Regulator Bypassing and Operation
If VIN is set to 10V, the power dissipation is: An internal voltage regulator produces the 6.9V supply PIC = 10
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27mA = 270mW that powers the gate drivers and logic circuitry within the and the junction temperature (assuming 70 degree ambi- LTC3806. The INTVCC regulator can supply up to 50mA ent temperature) is: and must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7μF ceramic capacitor. TJ = 70°C + 270mW
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120°C/W = 102.4°C Good bypassing is necessary to supply the high transient To prevent the maximum junction temperature from being currents required by the MOSFET gate drivers. exceeded, the input supply current must be checked when In an actual application, most of the IC supply current is operating at high VIN. If junction temperature is too high, used to drive the gate capacitances of the power MOSFETs. using a separate transformer winding to lower VIN may be As a result, high input voltage applications with large tried. Prior to adding an additional transformer winding power MOSFETs can cause the LTC3806 to exceed its (which raises transformer cost), be sure to check with maximum junction temperature rating. The junction tem- power MOSFET manufacturers for their newest low QG, perature can be estimated using the following equations: low RDS(ON) devices. Power MOSFET manufacturing tech- nologies are continually improving, with newer and better IQ(TOT) = IQ + f • QG performance devices being introduced almost yearly. PIC = VIN • (IQ + f • QG)
Output Voltage Programming
TJ = TA + PIC • RTH(JA) This IC will generally be used in DC/DC converters with where multiple outputs. The output voltage of the master output IQ is the static supply current (VOUT1) is set by a resistor divider according to the following formula: QG is the total gate charge of all external power MOSFETs PIC is the power dissipated in the IC ⎛ R6 ⎞ V = . V • + OUT1 1 230 1 f is the switching frequency, nominally 250kHz ⎝⎜ R7 ⎠⎟ RTH(JA) is the package thermal resistance, junction to The external resistor divider is connected as shown in ambient, nominally 34°C/W for the 12-pin DFN package Figure 1. The resistors R6 and R7 are typically chosen so that the error caused by the current flowing into the FB pin As an example, consider a 2-output power supply that during normal operation is less than 1% (this translates to uses an Si7450DP primary-side power MOSFET, that has a maximum value of R7 of about 120k). a maximum total gate charge of 42nC and two Si4840DY power MOSFETs (one for each output), each of which has The nominal slave output (VOUT2) voltage is set according 28nC maximum total gate charge. to the following formula: The total gate charge is: VOUT2 = VOUT1 • N21 QG = 42nC + 2
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28nC = 98nC where N21 is the turns ratio of the transformer windings between V The total supply current is: OUT2 and VOUT1. If additional slave outputs are added their voltage is IQ(TOT) = 2000μA + 98nC
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250kHz = 27mA determined by the equation: This demonstrates how significant the gate charge current V can be when compared to static quiescent current in the OUTN = VOUT1 • NN1 IC. where NN1 is the turns ratio of the transformer windings between VOUTN and VOUT1. 3806fb 9