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UC1845A-SP 数据表(PDF) 11 Page - Texas Instruments

部件名 UC1845A-SP
功能描述  UC1845A-SP QML Class V, Radiation Hardened Current-Mode PWM Controller
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制造商  TI [Texas Instruments]
网页  http://www.ti.com
标志 TI - Texas Instruments

UC1845A-SP 数据表(HTML) 11 Page - Texas Instruments

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UC1845A
UC1845A-SP
www.ti.com
SLUSC14 – MAY 2015
Feature Description (continued)
9.3.2 Reference
As highlighted in the Functional Block Diagram, UC1845A-SP incorporates a 5-V internal reference regulator with
±2% set point variation over temperature.
9.3.3 Totem-Pole Output
The UC1845A PWM has a single totem-pole output which can be operated to ±1-A peak for driving MOSFET
gates, and a +200 mA average current for bipolar power U-100A transistors. Cross conduction between the
output transistors is minimal, the average added power with VIN = 30 V is only 80 mW at 200 kHz.
Limiting the peak current through the IC is accomplished by placing a resistor between the totem-pole output and
the gate of the MOSFET. The value is determined by dividing the totem-pole collector voltage VC by the peak
current rating of the IC’s totem-pole. Without this resistor, the peak current is limited only by the dV/dT rate of the
totem-pole switching and the FET gate capacitance.
The use of a Schottky diode from the PWM output to ground prevents the output voltage from going excessively
below ground, causing instabilities within the IC. To be effective, the diode selected should have a forward drop
of less than 0.3 V at 200 mA. Most 1- to 3-A Schottky diodes exhibit these traits above room temperature.
Placing the diode as physically close to the PWM as possible enhances circuit performance. Implementation of
the complete drive scheme is shown in Figure 8 through Figure 10. Transformer-driven circuits also require the
use of the Schottky diodes to prevent a similar set of circumstances from occurring on the PWM output. The
ringing below ground is greatly enhanced by the transformer leakage inductance and parasitic capacitance, in
addition to the magnetizing inductance and FET gate capacitance. Circuit implementation is similar to the
previous example.
Figure 8 through Figure 10 show suggested circuits for driving MOSFETs and bipolar transistors with the
UC1845A output. The simple circuit of Figure 8 can be used when the control IC is not electrically isolated from
the MOSFET turn-on and turn-off to ±1 A. It also provides damping for a parasitic tank circuit formed by the FET
input capacitance and series wiring inductance. Schottky diode, D1, prevents the output of the IC from going far
below ground during turn-off.
Figure 9 shows an isolated MOSFET drive circuit which is appropriate when the drive signal must be level shifted
or transmitted across an isolation boundary. Bipolar transistors can be driven efficiently with the circuit of
Figure 10. Resistors R1 and R2 fix the on-state base current while capacitor Cl provides a negative base current
pulse to remove stored charge at turn-off.
Because the UC1845A series has only a single output, an interface circuit is needed to control push-pull half-
bridge or full-bridge topologies. The UC1706 dual output driver with internal toggle flip-flop performs this function.
Typical Application shows a typical application for these two ICs. Increased drive capability for driving numerous
FETs in parallel, or other loads can be accomplished using one of the UC1705/6/7 driver ICs.
Figure 8. Direct MOSFET Drive
Copyright © 2015, Texas Instruments Incorporated
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