Datasheet LT1160, LT1162 (Analog Devices) - 10

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