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PDF LM25085A Data sheet ( Hoja de datos )
| Número de pieza | LM25085A | |
| Descripción | 42V Constant On-Time PFET Buck Switching Controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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LM25085A
March 16, 2009
42V Constant On-Time PFET Buck Switching Controller
with 0.9V Reference
General Description
The LM25085A is a functional variant of the LM25085 COT
PFET Buck Switching Controller. The functional differences
of the LM25085A are: The feedback reference voltage is
0.9V, the forced off-time after current limit detection is longer,
and the soft-start time is shorter (1.8 ms).
The LM25085A is a high efficiency PFET switching regulator
controller that can be used to quickly and easily develop a
small, efficient buck regulator for a wide range of applications.
This high voltage controller contains a PFET gate driver and
a high voltage bias regulator which operates over a wide 4.5V
to 42V input range. The constant on-time regulation principle
requires no loop compensation, simplifies circuit implemen-
tation, and results in ultra-fast load transient response. The
operating frequency remains nearly constant with line and
load variations due to the inverse relationship between the
input voltage and the on-time. The PFET architecture allows
100% duty cycle operation for a low dropout voltage. Either
the RDS(ON) of the PFET or an external sense resistor can be
used to sense current for over-current detection.
Features
■ Wide 4.5V to 42V input voltage range
■ Adjustable current limit using RDS(ON) or a current sense
resistor
■ Programmable switching frequency to 1MHz
■ No loop compensation required
■ Ultra-Fast transient response
■ Nearly constant operating frequency with line and load
variations
■ Adjustable output voltage from 0.9V
■ Precision ±2% feedback reference
■ Capable of 100% duty cycle operation
■ Internal soft-start timer
■ Integrated high voltage bias regulator
■ Thermal shutdown
Package
■ MSOP-8EP
■ MSOP-8
■ LLP-8 (3 mm x 3 mm)
Typical Application, Basic Step Down Controller
© 2009 National Semiconductor Corporation 300852
30085201
www.national.com
1 page  
Symbol
Parameter
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Conditions
tOFF(CL1)
Off-time (current limit) (Note 7) VIN = 12V, VFB = 0V
tOFF(CL2)
VIN = 12V, VFB = 0.75V
tOFF(CL3)
VIN = 24V, VFB = 0V
tOFF(CL4)
VIN = 24V, VFB = 0.75V
Regulation and Over-Voltage Comparators (FB Pin)
VREF
FB regulation threshold
VOV FB over-voltage threshold
IFB FB bias current
Soft-Start Function
Measured with respect to VREF
tSS Soft-start time
Thermal Shutdown
TSD Junction shutdown temperature
THYS
Junction shutdown hysteresis
Thermal Resistance
Junction temperature rising
θJA Junction to ambient, 0 LFPM air MSOP-8 package
flow (Note 5)
MSOP-8EP package
LLP-8 package
θJC Junction to case, 0 LFPM air flow MSOP-8 package
(Note 5)
MSOP-8EP package
LLP-8 package
Min Typ Max Units
5.56
2.59
9.03
4.29
8 10.96
3.7 5.16
13.2 18.1
6 8.54
µs
µs
µs
µs
0.882
0.9 0.918 V
350 mV
10 nA
1.16 1.8 3.15 ms
170 °C
20 °C
126 °C/W
46
54
29 °C/W
5.5
9.1
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 3: VCC provides self bias for the internal gate drive.
Note 4: For detailed information on soldering plastic MSOP and LLP packages refer to the Packaging Data Book available from National Semiconductor
Corporation.
Note 5: Tested on a 4 layer JEDEC board. Four vias provided under the exposed pad. See JEDEC standards JESD51-5 and JESD51-7.
Note 6: Operating current and shutdown current do not include the current in the RT resistor.
Note 7: The tolerance of the minimum on-time (tON-4) and the current limit off-times (tOFF(CL1) through (tOFF(CL4)) track each other over process and temperature
variations. A device which has an on-time at the high end of the range will have an off-time that is at the high end of its range.
5 www.national.com
5 Page 
wwFwu.DnatcaSthioeent4Ua.lcoDm escription
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 out-
put 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 in-
versely 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 ap-
proximately 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 refer-
ence voltage (0.9V). When the FB pin voltage falls below the
feedback reference, Q1 is switched on for a time period de-
termined 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 re-
sistors (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 opera-
tion 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 re-
sistors. 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 dis-
continuous conduction mode at light load currents. In contin-
uous conduction mode the inductor’s current is always
greater than zero, and the operating frequency remains rela-
tively 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 conduc-
tion mode, where the inductor’s current reaches zero during
the off-time, the operating frequency is lower than in contin-
uous conduction mode and varies with load current. Conver-
sion 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-volt-
age comparator immediately switches off Q1. The next on-
time period starts when the voltage at FB falls below the
feedback reference voltage.
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 cal-
culated 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 recommend-
ed.
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:
11 www.national.com
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LM25085A.PDF ] | |
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