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LM25085A Folha de dados(PDF) 10 Page - Texas Instruments

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Nome de Peças LM25085A
Descrição Electrónicos  42V Constant On-Time PFET Buck Switching Controller with 0.9V Reference
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Fabricante Electrônico  TI1 [Texas Instruments]
Página de início  http://www.ti.com
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Duty Cycle =
tON
tON + tOFF
=
= tON x FS
VOUT
VIN
LM25085A
SNVS601A – JANUARY 2009 – REVISED MARCH 2009
www.ti.com
Functional Description
OVERVIEW
The LM25085A is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle.
The input operating voltage range of the LM25085A is 4.5V to 42V. The use of a PFET in a buck regulator
greatly simplifies the gate drive requirements and allows for 100% duty cycle operation to extend the regulation
range when operating at low input voltage. However, PFET transistors typically have higher on-resistance and
gate charge when compared to similarly rated NFET transistors. Consideration of available PFETs, input voltage
range, gate drive capability of the LM25085A, and thermal resistances indicate an upper limit of 10A for the load
current for LM25085A applications. Constant on-time control is implemented using an on-time one-shot that is
triggered by the feedback signal. During the off-time, when the PFET (Q1) is off, the load current is supplied by
the inductor and the output capacitor. As the output voltage falls, the voltage at the feedback comparator input
(FB) falls below the regulation threshold. When this occurs Q1 is turned on for the one-shot period which is
determined by the input voltage (VIN) and the RT resistor. During the on-time the increasing inductor current
increases the voltage at FB above the feedback comparator threshold. For a buck regulator the basic relationship
between the on-time, off-time, input voltage and output voltage is:
(1)
where Fs is the switching frequency. Equation 1 is valid only in continuous conduction mode (inductor current
does not reach zero). Since the LM25085A controls the on-time inversely proportional to VIN, the switching
frequency remains relatively constant as VIN is varied. If the input voltage falls to a level that is equal to or less
than the regulated output voltage Q1 is held on continuously (100% duty cycle) and VOUT is approximately equal
to VIN.
The COT control scheme, with the feedback signal applied to a comparator rather than an error amplifier,
requires no loop compensation, resulting in very fast load transient response.
The LM25085A is available in both an 8 pin MSOP package and an 8 pin LLP package with an exposed pad to
aid in heat dissipation. An 8 pin MSOP package without an exposed pad is also available.
REGULATION CONTROL CIRCUIT
The LM25085A buck DC-DC controller employs a control scheme based on a comparator and a one-shot on-
timer, with the output voltage feedback compared to an internal reference voltage (0.9V). When the FB pin
voltage falls below the feedback reference, Q1 is switched on for a time period determined by the input voltage
and a programming resistor (RT). Following the on-time Q1 remains off until the FB voltage falls below the
reference. Q1 is then switched on for another on-time period. The output voltage is set by the feedback resistors
(RFB1, RFB2 in the Block Diagram). The regulated output voltage is calculated as follows:
VOUT = 0.9V x (RFB2+ RFB1)/ RFB1
(2)
The feedback voltage supplied to the FB pin is applied to a comparator rather than a linear amplifier. For proper
operation sufficient ripple amplitude is necessary at the FB pin to switch the comparator at regular intervals with
minimum delay and noise susceptibility. This ripple is normally obtained from the output voltage ripple attenuated
through the feedback resistors. The output voltage ripple is a result of the inductor’s ripple current passing
through the output capacitor’s ESR, or through a resistor in series with the output capacitor. Multiple methods are
available to ensure sufficient ripple is supplied to the FB pin, and three different configurations are discussed in
the Applications Information section.
When in regulation, the LM25085A operates in continuous conduction mode at medium to 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. In
discontinuous conduction mode, where the inductor’s current reaches zero during the off-time, the operating
frequency is lower than in continuous conduction mode and varies with load current. Conversion efficiency is
maintained at light loads since the switching losses are reduced with the reduction in load and frequency.
If the voltage at the FB pin exceeds 1.25V due to a transient overshoot or excessive ripple at VOUT the internal
over-voltage comparator immediately switches off Q1. The next on-time period starts when the voltage at FB falls
below the feedback reference voltage.
10
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