LM5007MM National Semiconductor, LM5007MM Datasheet - Page 6

• Manufacturer Part Number: LM5007MM
• Description: IC BUCK ADJ .5A 8MSOP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM5007MM

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 2.5 ~ 73 V
• Current - Output: 500mA
• Frequency - Switching: 800kHz
• Voltage - Input: 9 ~ 75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• For Use With: LM5007EVAL - EVALUATION BOARD FOR LM5007
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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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 provid-
ing very fast load transient response. An intelligent current
limit scheme is implemented in the LM5007 with forced off
time, after current limit detection, which is inversely propor-
tional to Vout. This current limiting scheme reduces load
current foldback. Additional protection features include: Ther-
mal Shutdown, Vcc undervoltage lockout, Gate drive under-
voltage lockout and Max Duty Cycle limiter. The LM5007 can
be applied in numerous applications to efficiently regulate
step down higher voltage inputs. This regulator 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 be-
low the reference voltage, the buck switch is turned on for a
fixed time pulse determined by the line voltage and a pro-
gramming resistor (R
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. Therefore
at light loads the conversion efficiency is maintained, since
the switching losses decrease with the reduction in load cur-
rent and switching frequency. The approximate discontinuous
mode operating frequency can be calculated as follows:
ON
). Following the on period the switch
6
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 oper-
ating frequency can be calculated as follows:
The output voltage (Vout) can be programmed by two external
resistors as shown in Figure 1. The regulation point can be
calculated as follows:
The feedback comparator in hysteretic regulators depend up-
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 Equiva-
lent Series Resistance (ESR) is required. A ripple voltage of
25mV to 50mV 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.
For applications where lower output voltage ripple is required
the load can be connected directly to the low ESR output ca-
pacitor, as shown in Figure 1. The series resistor (R) will
degrade the load regulation. Another technique for enhancing
the ripple voltage at the FB pin is to place a capacitor in par-
allel with the feedback divider resistor R1. The addition of the
capacitor reduces the attenuation of the ripple voltage from
the feedback divider
High Voltage Bias Regulator
The LM5007 contains an internal high voltage bias regulator.
The input pin (Vin) can be connected directly to line voltages
from 9 to 75 Volts. To avoid supply voltage transients due to
long lead inductances on the input pin (Vin Pin 8), it is always
recommended to connect low ESR ceramic chip capacitor
(
close to LM5007. The regulator is internally current limited to
10mA. Upon power up, the regulator is enabled and sources
current into an external capacitor connected to the Vcc pin.
When the voltage on the Vcc pin reaches the regulation point
of 7V, the controller output is enabled.
An external auxiliary supply voltage can be applied to the Vcc
pin. If the auxiliary voltage is greater than 7 Volts the internal
regulator will essentially shutoff, thus reducing internal power
dissipation.
≊
0.1µF) between "Vin" pin and "RTN" pin (pin 4), located
V
OUT
= 2.5 x (R1 + R2) / R2