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LM25011 Folha de dados(PDF) 10 Page - Texas Instruments

Nome de Peças LM25011
Descrição Electrónicos  42V 2A Constant On-Time Switching Regulator With Adjustable Current Limit
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Fabricante Electrônico  TI1 [Texas Instruments]
Página de início  http://www.ti.com
Logo TI1 - Texas Instruments

LM25011 Folha de dados(HTML) 10 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
SNVS617F – MAY 2009 – REVISED JANUARY 2011
www.ti.com
CONTROL CIRCUIT OVERVIEW
The LM25011 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with
the output voltage feedback (FB) compared to an internal reference (2.51V). If the FB voltage is below the
reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls
below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the
FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned
on for another on-time one-shot period.
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’s 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 the inductor’s ripple current amplitude.
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 the inductor’s ripple current amplitude 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 since 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 circuit’s inductor.
The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Block Diagram). The regulated output
voltage is calculated as follows:
VOUT = 2.51V x (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 Applications Information
section.
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)
10
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Product Folder Links: LM25011 LM25011-Q1


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