LM5005EVAL National Semiconductor, LM5005EVAL Datasheet - Page 9

• Manufacturer Part Number: LM5005EVAL
• Description: BOARD EVALUATION LM5005
• Manufacturer: National Semiconductor

Specifications of LM5005EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 2.5A
• Voltage - Input: 7 ~ 75V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5005
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

Detailed Operating Description
The LM5005 high voltage switching regulator features all of
the functions necessary to implement an efficient high voltage
buck regulator using a minimum of external components. 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 current 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 processing 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 regula-
tors to self synchronize or be synchronized to an external
clock. The output voltage can be set from 1.225V. Fault pro-
tection 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 unreg-
ulated input voltage. The device is well suited for telecom,
industrial and automotive power bus voltage ranges.
High Voltage Start-Up Regulator
The LM5005 contains a dual mode internal high voltage start-
up regulator that provides the Vcc bias supply for the PWM
FIGURE 2. Vin and Vcc Sequencing
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controller and boot-strap MOSFET gate driver. The 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 ap-
proximately equal to Vin. For Vin voltage greater than 9V, the
low dropout switch is disabled and the Vcc regulator is en-
abled to maintain Vcc at approximately 7V. The wide operat-
ing 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 capac-
itor 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. There-
fore the auxiliary Vcc voltage should never exceed the Vin
voltage.
In high voltage applications extra care should be taken to en-
sure 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.
20161904
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