Datasheet
Duty Cycle =
t
ON
t
ON
+ t
OFF
=
= t
ON
x F
S
V
OUT
V
IN
-
+
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
V
IN
GND
SHUTDOWN
R
T
C
VCC
C
ADJ
R
ADJ
Q1
D1
L1
1.25V
R
FB1
R
FB2
C
BYP
R
SEN
VCC
V
OUT
C
OUT
R3
C1
C2
1.6V
+
-
+
-
7.7V
40 PA
+
-
0.73V
+
-
RT
C
IN
V
IN
V
IN
V
IN
LM25085, LM25085-Q1
www.ti.com
SNVS593H –OCTOBER 2008–REVISED MARCH 2013
Block Diagram
Sense resistor method shown for current limit detection.
Minimum output ripple configuration shown.
FUNCTIONAL DESCRIPTION
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 (V
IN
) and the R
T
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 V
IN
, the switching frequency remains relatively constant
as V
IN
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 V
OUT
is approximately equal to V
IN
.
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.
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM25085 LM25085-Q1