LM5005MH National Semiconductor, LM5005MH Datasheet - Page 10

• Manufacturer Part Number: LM5005MH
• Description: IC, BUCK REGULATOR, TSSOP-20
• Manufacturer: National Semiconductor

Specifications of LM5005MH

• Primary Input Voltage: 75V
• No. Of Outputs: 1
• Output Voltage: 1.225V
• Output Current: 2.5A
• No. Of Pins: 20
• Operating Temperature Range: -40°C To +125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Detailed Operating Description
The LM5005 high voltage switching regulator features all of
the functions necessary to implement an efficient high volt-
age buck regulator using a minimum of external compo-
nents. This easy to use regulator integrates a 75V
N-Channel buck switch with an output current capability of
2.5 Amps. The regulator control method is based on current
mode control utilizing an emulated current ramp. Peak cur-
rent mode control provides inherent line feed-forward, cycle-
by-cycle current limiting and ease of loop compensation. The
use of an emulated control ramp reduces noise sensitivity of
the pulse-width modulation circuit, allowing reliable process-
ing of very small duty cycles necessary in high input voltage
applications. The operating frequency is user programmable
from 50kHz to 500kHz. An oscillator synchronization pin
allows multiple LM5005 regulators to self synchronize or be
synchronized to an external clock. The output voltage can be
set from 1.225V. Fault protection features include, current
limiting, thermal shutdown and remote shutdown capability.
The device is available in the TSSOP-20 package featuring
an exposed pad to aid thermal dissipation.
The functional block diagram and typical application of the
LM5005 is shown in Figure 1. The LM5005 can be applied in
numerous applications to efficiently step-down a high un-
regulated input voltage. The device is well suited for tele-
com, industrial and automotive power bus voltage ranges.
High Voltage Start-Up Regulator
The LM5005 contains a dual mode internal high voltage
startup regulator that provides the Vcc bias supply for the
PWM controller and boot-strap MOSFET gate driver. The
FIGURE 2. Vin and Vcc Sequencing
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input pin (Vin) can be connected directly to the input voltage,
as high as 75 Volts. For input voltages below 9V, a low
dropout switch connects Vcc directly to Vin. In this supply
range, Vcc is approximately equal to Vin. For Vin voltage
greater than 9V, the low dropout switch is disabled and the
Vcc regulator is enabled to maintain Vcc at approximately
7V. The wide operating range of 7 to 75V is achieved through
the use of this dual mode regulator.
The output of the Vcc regulator is current limited to 20mA.
Upon power up, the regulator sources current into the ca-
pacitor connected to the Vcc pin. When the voltage at the
Vcc pin exceeds the Vcc UVLO threshold of 6.3V and the SD
pin is greater than 1.225V, the output switch is enabled and
a soft-start sequence begins. The output switch remains
enabled until Vcc falls below 5.3V or the SD pin falls below
1.125V.
An auxiliary supply voltage can be applied to the Vcc pin to
reduce the IC power dissipation. If the auxiliary voltage is
greater than 7.3V, the internal regulator will essentially shut-
off, reducing the IC power dissipation. The Vcc regulator
series pass transistor includes a diode between Vcc and Vin
that should not be forward biased in normal operation.
Therefore the auxiliary Vcc voltage should never exceed the
Vin voltage.
In high voltage applications extra care should be taken to
ensure the Vin pin does not exceed the absolute maximum
voltage rating of 76V. During line or load transients, voltage
ringing on the Vin line that exceeds the Absolute Maximum
Ratings can damage the IC. Both careful PC board layout
and the use of quality bypass capacitors located close to the
Vin and GND pins are essential.
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