Datasheet

F
S
=
V
OUT
2
x L1 x 1.4 x 10
20
R
L
x R
ON
2
DC =
t
ON
t
ON
+ t
OFF
V
OUT
V
IN
= t
ON
x F
S
=
F
S
=
V
OUT
x (V
IN
± 1.4V)
1.18 x 10
-10
x (R
ON
+ 1.4 k:) x V
IN
LM25010
SNVS419D –DECEMBER 2005–REVISED FEBRUARY 2013 www.ti.com
Functional Description
The LM25010 Step-Down Switching Regulator features all the functions needed to implement a low cost, efficient
buck DC-DC converter capable of supplying in excess of 1A to the load. This high voltage regulator integrates an
N-Channel buck switch, with an easy to implement constant on-time controller. It is available in the thermally
enhanced WSON-10 and HTSSOP-14EP packages. The regulator compares the feedback voltage to a 2.5V
reference to control the buck switch, and provides a switch on-time which varies inversely with VIN. This feature
results in the operating frequency remaining relatively constant with load and input voltage variations. The
switching frequency can range from less than 100 kHz to 1.0 MHz. The regulator requires no loop compensation
resulting in very fast load transient response. The valley current limit circuit holds the buck switch off until the
free-wheeling inductor current falls below the current limit threshold, nominally set at 1.25A.
The LM25010 can be applied in numerous applications to efficiently step-down higher DC voltages. Features
include: thermal shutdown, VCC under-voltage lock-out, gate drive under-voltage lock-out, and maximum duty
cycle limit.
Control Circuit Overview
The LM25010 employs a control scheme based on a comparator and a one-shot on-timer, with the output
voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below the reference the
buck switch is turned on for a time period determined by the input voltage and a programming resistor (R
ON
).
Following the on-time the switch remains off for a fixed 260 ns off-time, or until the FB voltage falls below the
reference, whichever is longer. The buck switch then turns on for another on-time period. Referring to the Block
Diagram, the output voltage is set by R1 and R2. The regulated output voltage is calculated as follows:
V
OUT
= 2.5V × (R1 + R2) / R2 (1)
The LM25010 requires a minimum of 25 mV of ripple voltage at the FB pin for stable fixed-frequency operation. If
the output capacitor’s ESR is insufficient additional series resistance may be required (R3 in the Block Diagram).
The LM25010 operates in continuous conduction mode at heavy load currents, and discontinuous conduction
mode at light load currents. In continuous conduction mode current always flows through the inductor, never
decaying to zero during the off-time. In this mode 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 operating frequency in the continuous conduction mode is calculated as follows:
(2)
The buck switch duty cycle is equal to:
(3)
Under light load conditions, the LM25010 operates in discontinuous conduction mode, with zero current flowing
through the inductor for a portion of the off-time. The operating frequency is always lower than that of the
continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is
maintained at a relatively high level at light loads since the switching losses diminish as the power delivered to
the load is reduced. The discontinuous mode operating frequency is approximately:
(4)
where R
L
= the load resistance.
Start-Up Bias Regulator (V
CC
)
A high voltage bias regulator is integrated within the LM25010. The input pin (VIN) can be connected directly to
line voltages between 6V and 42V. Referring to the Block Diagram and the graph of V
CC
vs. V
IN
, when V
IN
is
between 6V and the bypass threshold (nominally 8.9V), the bypass switch (Q2) is on, and V
CC
tracks V
IN
within
100 mV to 150 mV. The bypass switch on-resistance is approximately 50Ω, with inherent current limiting at
approximately 100 mA. When VIN is above the bypass threshold, Q2 is turned off, and V
CC
is regulated at 7V.
The V
CC
regulator output current is limited at approximately 15 mA. When the LM25010 is shutdown using the
RON/SD pin, the V
CC
bypass switch is shut off, regardless of the voltage at VIN.
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