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LM25011A-Q1 Datenblatt(PDF) 11 Page - Texas Instruments

Teilenummer LM25011A-Q1
Bauteilbeschribung  LM25011, LM25011-Q1, LM25011A, LM25011A-Q1 42-V 2-A Constant On-Time Switching Regulator With Adjustable Current Limit
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Hersteller  TI [Texas Instruments]
Direct Link  http://www.ti.com
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LM25011A-Q1 Datenblatt(HTML) 11 Page - Texas Instruments

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RT =
VOUT - (VIN x FS x 15 ns)
FS x 4.1 x 10
-11
- 500:
tON =
(VIN)
4.1 x 10
-11
x (RT + 500:)
+ 15 ns
FS =
VOUT
2 x L1 x 1.19 x 1021
RL x RT
2
DC =
tON
tON + tOFF
=
= tON x FS
VOUT
VIN
FS =
VOUT
(4.1 x 10
-11
x (RT + 0.5k)) + (VIN x 15 ns)
LM25011, LM25011-Q1, LM25011A, LM25011A-Q1
www.ti.com
SNVS617H – APRIL 2009 – REVISED NOVEMBER 2014
Feature Description (continued)
When in regulation, the LM25011 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode, the inductor current is
always greater than zero and the operating frequency remains relatively constant with load and line variations.
The minimum load current for continuous conduction mode is one-half of the ripple current amplitude of the
inductor. The approximate operating frequency is calculated as follows:
(1)
The buck switch duty cycle is approximately equal to:
(2)
When the load current is less than one-half of the ripple current amplitude of the inductor, the circuit operates in
discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor
current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is
maintained at light loads because the switching losses are reduced with the reduction in load and frequency. The
approximate discontinuous operating frequency can be calculated as follows:
(3)
where RL = the load resistance, and L1 is the inductor in the circuit.
The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Functional Block Diagram ). The
regulated output voltage is calculated as follows:
VOUT = 2.51 V × (RFB1 + RFB2) / RFB1
(4)
Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is
generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM
(emulated ripple mode) control circuit generates the required internal ripple voltage from the ripple waveform at
the CS pin. The LM25011A, which is designed for higher frequency operation, requires additional ripple voltage
which must be generated externally and provided to the FB pin. This is described in the Application and
Implementation section.
7.3.2 On-Time Timer
The on-time for the LM25011/LM25011A is determined by the RT resistor and the input voltage (VIN), calculated
from:
(5)
The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific
continuous conduction mode switching frequency (FS), the RT resistor is determined from the following:
(6)
The on-time must be chosen greater than 90 ns for proper operation. Equation 1, Equation 5, and Equation 6 are
valid only during normal operation; that is, the circuit is not in current limit. When the LM25011 operates in
current limit, the on-time is reduced by approximately 40% (this feature is not present in LM25011A). This feature
reduces the peak inductor current which may be excessively high if the load current and the input voltage are
simultaneously high. This feature operates on a cycle-by-cycle basis until the load current is reduced and the
Copyright © 2009–2014, Texas Instruments Incorporated
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