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LT1160IN Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LT1160IN Datasheet(HTML) 10 Page - Linear Integrated Systems |
10 / 16 page 10 LT1160/LT1162 11602fb APPLICATIONS INFORMATION The actual increase in supply current is slightly higher due to LT1160 switching losses and the fact that the gates are being charged to more than 10V. Supply Current vs Input Frequency is given in the Typical Performance Characteristics. The LT1160 junction temperature can be estimated by using the equations given in Note 2 of the Electrical Characteristics. For example, the LT1160IS is limited to less than 31mA from a 12V supply: TJ = 85°C + (31mA)(12V)(110°C/W) = 126 °C exceeds absolute maximum In order to prevent the maximum junction temperature from being exceeded, the LT1160 supply current must be verified while driving the full complement of the chosen MOSFET type at the maximum switching frequency. Ugly Transient Issues In PWM applications the drain current of the top MOSFET is a square wave at the input frequency and duty cycle. To prevent large voltage transients at the top drain, a low ESR electrolytic capacitor must be used and returned to the power ground. The capacitor is generally in the range of 25 µF to 5000µF and must be physically sized for the RMS current flowing in the drain to prevent heating and prema- ture failure. In addition, the LT1160 requires a separate 10 µF capacitor connected closely between Pins 1 and 5 (the LT1162 requires two 10 µF capacitors connected between Pins 1 and 5, and Pins 7 and 11). The LT1160 top source is internally protected against transients below ground and above supply. However, the gate drive pins cannot be forced below ground. In most applications, negative transients coupled from the source to the gate of the top MOSFET do not cause any problems. Switching Regulator Applications The LT1160 (or 1/2 LT1162) is ideal as a synchronous switch driver to improve the efficiency of step-down (buck) switching regulators. Most step-down regulators use a high current Schottky diode to conduct the inductor current when the switch is off. The fractions of the oscil- lator period that the switch is on (switch conducting) and off (diode conducting) are given by: Switch Total Period Switch Total Period ON = V HV OFF = HV – V HV OUT OUT ⎛ ⎝⎜ ⎞ ⎠⎟() ⎛ ⎝⎜ ⎞ ⎠⎟() Note that for HV > 2VOUT the switch is off longer than it is on, making the diode losses more significant than the switch. The worst case for the diode is during a short circuit, when VOUT approaches zero and the diode con- ducts the short-circuit current almost continuously. Figure 2 shows the LT1160 used to synchronously drive a pair of power MOSFETs in a step-down regulator applica- tion, where the top MOSFET is the switch and the bottom MOSFET replaces the Schottky diode. Since both conduc- tion paths have low losses, this approach can result in very high efficiency (90% to 95%) in most applications. For regulators under 10A, using low RDS(ON) N-channel MOSFETs eliminates the need for heat sinks. RGS holds the top MOSFET off when HV is applied before the 12V supply. One fundamental difference in the operation of a step- down regulator with synchronous switching is that it never becomes discontinuous at light loads. The inductor cur- rent doesn’t stop ramping down when it reaches zero but actually reverses polarity resulting in a constant ripple current independent of load. This does not cause a signifi- cant efficiency loss (as might be expected) since the negative inductor current is returned to HV when the switch turns back on. However, I2R losses will occur under these conditions due to the recirculating currents. The LT1160 performs the synchronous MOSFET drive in a step-down switching regulator. A reference and PWM are required to complete the regulator. Any voltage mode or current mode PWM controller may be used but the LT3526 is particularly well-suited to high power, high efficiency applications such as the 10A circuit shown in Figure 4. In higher current regulators a small Schottky diode across the bottom MOSFET helps to reduce reverse-recovery switching losses. |
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