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LM25085 Folha de dados(PDF) 11 Page - Texas Instruments

Nome de Peças LM25085
Descrição Electrónicos  LM25085 / -Q1 42V Constant On-Time PFET Buck Switching Controller
Download  34 Pages
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Fabricante Electrônico  TI [Texas Instruments]
Página de início  http://www.ti.com
Logo TI - Texas Instruments

LM25085 Folha de dados(HTML) 11 Page - Texas Instruments

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background image
-
+
Q
R
+
S
Negative Bias
Regulator
Thermal
Shutdown
ON Time
One-Shot
Soft-Start
Gate Driver
Control Logic
VCC
UVLO
Gate
Driver
REGULATION
COMPARATOR
OVER-VOLTAGE
COMPARATOR
CURRENT
LIMIT
COMPARATOR
OFF Time
One-Shot
FB
LM25085
PGATE
ISEN
GND
VCC
ADJ
4.5V to 42V
Input
VIN
GND
SHUTDOWN
RT
CVCC
CADJ
RADJ
Q1
D1
L1
1.25V
RFB1
RFB2
CBYP
RSEN
VCC
VOUT
COUT
R3 C1
C2
1.6V
+
-
+
-
7.7V
40 2A
+
-
0.73V
+
-
RT
CIN
VIN
VIN
VIN
Duty Cycle =
tON
tON + tOFF
=
= tON x FS
VOUT
VIN
LM25085, LM25085-Q1
www.ti.com
SNVS593J – OCTOBER 2008 – REVISED NOVEMBER 2014
7 Detailed Description
7.1 Overview
The LM25085 is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle.
The input operating voltage range of the LM25085 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 LM25085, and thermal resistances indicate an upper limit of 10A for the load
current for LM25085 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:
where
Fs is the switching frequency
(1)
Equation 1 is valid only in continuous conduction mode (inductor current does not reach zero). Since the
LM25085 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 LM25085 is available in both an 8 pin HVSSOP-PowerPAD package and an 8 pin WSON package with an
exposed pad to aid in heat dissipation. An 8 pin VSSOP package without an exposed pad is also available.
7.2 Functional Block Diagram
Sense resistor method shown for current limit detection.
Minimum output ripple configuration shown.
Copyright © 2008–2014, Texas Instruments Incorporated
Submit Documentation Feedback
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