Specifications with standard typeface are for T

Temperature range.V

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Minimum Off Time

Minimum Off Timer

FB = 0V

300

ns

Regulation and OV Comparators

FB Reference Threshold

Internal reference

Trip point for switch ON

2.445

2.5

2.550

V

FB Over-Voltage Threshold

Trip point for switch OFF

2.875

V

FB Bias Current

100

nA

Thermal Shutdown

Tsd

Thermal Shutdown Temp.

165

˚C

Thermal Shutdown Hysteresis

25

˚C

Thermal Resistance

θ

JA

Junction to Ambient

MUA Package

200

˚C/W

SDC Package

40

˚C/W

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: For detailed information on soldering plastic MSOP and LLP packages, refer to the Packaging Data Book available from National Semiconductor

Corporation.

are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control.

Note 5: The human body model is a 100pF capacitor discharge through a 1.5k

Ω resistor into each pin. The machine model is a 200pF capacitor discharged directly

into each pin. The machine model ESD compliance level for Pin 5 is 150V. The human body ESD compliance level for Pin 7 and 8 is 1000V.

Note 6: For devices procurred in the LLP-8 package the Rds(on) limits are guaranteed by design characterization data only.

Detailed Operating Description

The LM5007 Step Down Switching Regulator features all of

the functions needed to implement low cost, efficient, Buck

bias regulators. This high voltage regulator contains an 80V,

0.7A N-Channel Buck Switch. The device is easy to apply

and is provided in the MSOP-8 and the thermally enhanced

LLP-8 packages. The regulator is based on a hysteretic

control scheme using an on time inversely proportional to

Vin. This feature allows the operating frequency to remain

relatively constant with load and input voltage variations. The

hysteretic control requires no control loop compensation,

while providing very fast load transient response. An intelli-

gent current limit scheme is implemented in the LM5007 with

forced off time, after current limit detection, which is in-

versely proportional to Vout. This current limiting scheme

reduces load current foldback. Additional protection features

include: Thermal Shutdown, Vcc undervoltage lockout, Gate

drive undervoltage lockout and Max Duty Cycle limiter. The

LM5007 can be applied in numerous applications to effi-

ciently regulate step down higher voltage inputs. This regu-

lator is well suited for 48 Volt Telcom and the new 42V

Automotive power bus ranges.

Hysteretic Control Circuit

Overview

The LM5007 is a Buck DC-DC regulator that uses an 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 LM5007 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 (R

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 LM5007 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 fre-

quency can be relatively low and will vary with load. There-

fore at light loads the conversion efficiency is maintained,

since the switching losses decrease with the reduction in

load current and switching frequency. The approximate dis-

continuous mode operating frequency can be calculated as

follows:

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 discontinu-

ous mode frequency and remains relatively constant with

load and line variations. The approximate continuous mode

operating frequency can be calculated as follows:

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