參數(shù)資料
型號(hào)： TPS40075RHLR
廠商： TEXAS INSTRUMENTS INC
元件分類： 穩(wěn)壓器
英文描述： 2 A SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PQCC20
封裝： PLASTIC, QFN-20
文件頁(yè)數(shù)： 10/44頁(yè)
文件大小： 806K
代理商： TPS40075RHLR
www.ti.com
PROGRAMMING SHORT CIRCUIT PROTECTION
ILIM Threshold
T2
T1
ILIM Threshold
T3
T1
ILIM
SW
VIN 2V
UDG03173
Overcurrent
(A)
(B)
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
The TPS40075 uses a two-tier approach to short circuit protection. The first tier is a pulse-by-pulse protection
scheme. Short circuit protection is implemented by sensing the voltage drop across the high-side MOSFET while
it is turned on. The MOSFET drain to source voltage is compared to the voltage dropped across a resistor (RILIM)
connected from VDD to the ILIM pin. The voltage drop across this resistor is produced by a constant current sink.
If the voltage drop across the MOSFET exceeds the voltage drop across the ILIM resistor the switching pulse is
immediately terminated. The MOSFET remains off until the next switching cycle is initiated.
In addition, just prior to the high-side MOSFET turning on, the ILIM pin is pulled down to approximately half of
VDD. The ILIM pin is allowed to return to its nominal value after one of two events occur:
1. The SW node rises to within approximately 2 V of VDD
2. An internal timeout occurs, approximately 125-ns after ILIM is initially pulled down
If the SW node rises to within approximately 2-V of VDD, the device allows ILIM to go back to its nominal value.
This is illustrated in Figure 27 A. T1 is the delay time from the internal PWM signal being asserted and the rise of
SW. This includes the driver delay of 50 ns typical, and the turn on time of the high-side MOSFET. The MOSFET
used should have a turn on time less than 75 ns. T2 is the reaction time of the sensing circuit that allows ILIM to
start to return to its nominal value, typically 20ns.
Figure 27. Switching and Current Limit Waveforms and Timing Relationship
The second event that can cause ILIM to return to its nominal value is for an internal timeout to expire. This is
illustrated in Figure 27 B as T3. Here SW never rises to VDD-2, for whatever reason, and the internal timer times
out. This allows the ILIM pin to start its transition back to its nominal value.
Prior to ILIM starting back to its nominal value, short circuit sensing is not enabled. In normal operation, this
insures that the SW node is at a higher voltage than ILIM when short circuit sensing starts, avoiding false trips
while allowing for a quicker blanking delay than would ordinarily be possible. Placing a capacitor across RILIM
sets an exponential approach to the normal voltage at the ILIM pin. This exponential “decay” of the short circuit
threshold can be used to compensate for ringing on the SW node after its rising edge and to help compensate
for slower turn-on MOSFETs. Choosing the proper capacitance requires care. If the capacitance is too large, the
voltage at ILIM does not approach the desired short circuit level quickly enough, resulting in an apparent shift in
short circuit threshold as pulse width changes.
18
Copyright 2006–2007, Texas Instruments Incorporated
Product Folder Link(s) :TPS40075
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