Datasheet LT1158 (Analog Devices) - 10

LT1158
APPLICATIONS INFORMATION
If individual gate decoupling resistors are used, the gate
MOSFET Gate Drive Protection
feedback pins can be connected to any one of the gates. For supply voltages of over 8V, the LT1158 will protect Driving multiple MOSFETs in parallel may restrict the standard N-channel MOSFETs from under or overvoltage operating frequency at high supply voltages to prevent gate drive conditions for any input duty cycle including over-dissipation in the LT1158 (see Gate Charge and DC. Gate-to-source Zener clamps are not required and Driver Dissipation below). When the total gate capacitance not recommended since they can reduce operating exceeds 10,000pF on the top side, the bootstrap capacitor efficiency. should be increased proportionally above 0.1μF. A discontinuity in tracking between the output pulse width and input pulse width may be noted as the top side
Gate Charge and Driver Dissipation
MOSFET approaches 100% duty cycle. As the input low A useful indicator of the load presented to the driver by a signal becomes narrower, it may become shorter than power MOSFET is the total gate charge QG, which includes the time required to recharge the bootstrap capacitor to the additional charge required by the gate-to-drain swing. QG a safe voltage for the top side driver. Below this duty cycle is usually specifi ed for VGS = 10V and VDS = 0.8VDS(MAX). the output pulse width will stop tracking the input until When the supply current is measured in a switching ap- the input low signal is <100ns, at which point the output plication, it will be larger than given by the DC electrical will jump to the DC condition of top MOSFET “on” and characteristics because of the additional supply current bottom MOSFET “off.” associated with sourcing the MOSFET gate charge:
Low Voltage Operation
dQ ⎞ dQ ⎞ I =I G G The LT1158 can operate from 5V supplies (4.5V min) and SUPPLY DC + ⎛ ⎝⎜ dt ⎠⎟ + ⎛⎝⎜ dt ⎠⎟ TOP BOTTOM in 6V battery-powered applications by using logic-level N-channel power MOSFETs. These MOSFETs have 2V The actual increase in supply current is slightly higher maximum threshold voltages and guaranteed RDS(ON) limits due to LT1158 switching losses and the fact that the gates at VGS = 4V. The switching speed of the LT1158, unlike are being charged to more than 10V. Supply current vs CMOS drivers, does not degrade at low supply voltages. switching frequency is given in the Typical Performance For operation down to 4.5V, the boost pin should be con- Characteristics. nected as shown in Figure 2 to maximize gate drive to the The LT1158 junction temperature can be estimated by top side MOSFET. Supply voltages over 10V should not using the equations given in Note 1 of the electrical char- be used with logic-level MOSFETs because of their lower acteristics. For example, the LT1158SI is limited to less maximum gate-to-source voltage rating. than 25mA from a 24V supply: 5V T N.C. J = 85°C + (25mA • 24V • 110°C/W) + = 151°C exceeds absolute maximum D1 BOOST DR BOOST In order to prevent the maximum junction temperature 0.1μF from being exceeded, the LT1158 supply current must T GATE DR LT1158 LOGIC-LEVEL be checked with the actual MOSFETs operating at the T GATE FB MOSFET maximum switching frequency. T SOURCE D1: LOW-LEAKAGE SCHOTTKY BAT85 OR EQUIVALENT LT1158 F02
Figure 2. Low Voltage Operation
1158fb 10