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LM25085A bảng dữ liệu(PDF) 11 Page - Texas Instruments

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Giải thích chi tiết về linh kiện  42V Constant On-Time PFET Buck Switching Controller with 0.9V Reference
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RT
Input
Voltage
STOP
RUN
LM25085A
RT
VIN
RT =
VOUT x 6 x 10
6
FS
- 8.6
FS =
VOUT x (VIN - 1.56V + RT/3167)
VIN x [(1.45 x 10
-7
x (RT + 1.4)) + (tD x (VIN - 1.56V + RT/3167))]
tON =
(VIN - 1.56V + RT/3167)
1.45 x 10
-7
x (RT + 1.4)
+ 50 ns
LM25085A
www.ti.com
SNVS601A – JANUARY 2009 – REVISED MARCH 2009
ON-TIME TIMER
The on-time of the PFET gate drive output (PGATE pin) is determined by the resistor (RT) and the input voltage
(VIN), and is calculated from:
(3)
where RT is in kΩ.
The minimum on-time, which occurs at maximum VIN, should not be set less than 150 ns (see Current Limiting
section). The buck regulator effective on-time, measured at the SW node (junction of Q1, L1, and D1) is typically
longer than that calculated in Equation 3 due to the asymmetric delay of the PFET. The on-time difference
caused by the PFET switching delay can be estimated as the difference of the turn-off and turn-on delays listed
in the PFET data sheet. Measuring the difference between the on-time at the PGATE pin versus the SW node in
the actual application circuit is also recommended.
In continuous conduction mode, the inverse relationship of tON with VIN results in a nearly constant switching
frequency as VIN is varied. The operating frequency can be calculated from:
(4)
where RT is in kΩ, and tD is equal to 50 ns plus the PFET’s delay difference. To set a specific continuous
conduction mode switching frequency (FS), the RT resistor is determined from the following:
(5)
where RT is in kΩ. A simplified version of Equation 5 at VIN = 12V, and tD = 100 ns, is:
(6)
For VIN = 42V and tD = 100 ns, the simplified equation is:
(7)
SHUTDOWN
The LM25085A can be shutdown by grounding the RT pin (see Figure 4). In this mode the PFET is held off, and
the VCC regulator is disabled. The internal operating current is reduced to the value shown in the graph
“Shutdown current vs. VIN”. The shutdown threshold at the RT pin is
≊0.73V, with ≊50 mV of hysteresis.
Releasing the pin enables normal operation. The RT pin must not be forced high during normal operation.
Figure 4. Shutdown Implementation
CURRENT LIMITING
The LM25085A current limiting operates by sensing the voltage across either the RDS(ON) of Q1, or a sense
resistor, during the on-time and comparing it to the voltage across the resistor RADJ (see Figure 5). The current
limit function is much more accurate and stable over temperature when a sense resistor is used. The RDS(ON) of a
MOSFET has a wide process variation and a large temperature coefficient.
Copyright © 2009, Texas Instruments Incorporated
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