F =

VOUT

1.42 x 10

-10 x R

ON

F =

VOUT

2 x L

1 x 10

-20 x R

Load x (RON)

2

LM25007

www.ti.com

SNVS401C – JANUARY 2006 – REVISED FEBRUARY 2013

DETAILED OPERATING DESCRIPTION

The LM25007 is a monolithic step-down switching regulator featuring all of the functions needed to implement

low cost, efficient, power supply. Capable of driving a 0.5Amp load over a 9-42V input voltage range, this device

is easy to apply and is provided in the small VSSOP-8 and the thermally enhanced WSON-8 packages. Ultra-fast

transient response is achieved through the use of a constant on-time architecture with Vin feed forward. This

feature provides and almost constant operating frequency over load and input voltage variations. The operating

frequency may be adjusted up to 800kHz depending on input and output voltages. This architecture is easy to

use and tolerant of component selection. An intelligent current limit is implemented in the LM25007 with forced

off time that is inversely proportional to Vout to ensure recovery from fault conditions. The output voltage may be

set from 2.5V to >30V. To improve efficiency in light load applications, the Vcc pin can be tied to an external

voltage source thus eliminating the drop through the integrated internal regulator. Additional protection features

include: thermal shutdown, Vcc under-voltage lockout, gate drive under-voltage lockout, and max duty cycle

limiter.

Hysteretic Control Circuit Overview

The LM25007 is a Buck DC-DC regulator that uses a constant on-time control scheme. The on-time is

programmed by an external resistor and varies inversely with line input voltage (Vin). The core regulation

elements of the LM25007 are the feedback comparator and the on-time one-shot. The regulator output voltage is

sensed at the feedback pin (FB) and is compared to an internal reference voltage (2.5V). If the FB signal is

below the reference voltage, the buck switch is turned on for a fixed time pulse determined by the line voltage

and a programming resistor (RON). Following the on period the switch will remain off for at least the minimum off

timer period of 300ns. If the FB pin voltage is still below the reference after the 300ns off-time, the switch will turn

on again for another on time period. This switching behavior will continue until the FB pin voltage reaches the

reference voltage level.

The LM25007 operates in discontinuous conduction mode at light load currents or continuous conduction mode

at heavier load currents. In discontinuous conduction mode, current through the output inductor starts at zero and

ramps up to a peak value during the buck switch on-time and then back to zero during the off-time. The inductor

current remains at zero until the next on-time period starts when FB falls below the internal reference. In

discontinuous mode the operating frequency is relatively low and will vary with load. Therefore at light loads the

conversion efficiency is maintained, since the switching losses decrease with the reduction in load current and

switching frequency. The approximate discontinuous mode operating frequency can be calculated as follows:

(1)

In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.

In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively

constant with load and line variations. The approximate continuous mode operating frequency can be calculated

as follows:

(2)

The output voltage (Vout) can be programmed by two external resistors as shown in Figure 6. The regulated

voltage is calculated as follows:

VOUT = 2.5 x (R3 + R4) / R4

(3)

The feedback comparator in hysteretic regulators depends on the output ripple voltage to switch the output

transistor on and off at regular intervals. In order for the internal comparator to respond quickly to changes in

output voltage, proportional to inductor current, a minimum amount of capacitor Equivalent Series Resistance

(ESR) is required. A ripple voltage of 25mV is recommended at the feedback pin (FB) for stable operation. In

cases where the intrinsic capacitor ESR is too small, additional series resistance may be added.

Copyright © 2006–2013, Texas Instruments Incorporated

Submit Documentation Feedback

7

Product Folder Links: LM25007