lm25576q0 National Semiconductor Corporation, lm25576q0 Datasheet - Page 9

• Manufacturer Part Number: lm25576q0
• Description: Simple Switcher 42v, 3a Step-down Switching Regulator
• Manufacturer: National Semiconductor Corporation

Detailed Operating Description
The LM25576Q0 switching regulator features all of the func-
tions necessary to implement an efficient high voltage buck
regulator using a minimum of external components. This easy
to use regulator integrates a 42V N-Channel buck switch with
an output current capability of 3 Amps. The regulator control
method is based on current mode control utilizing an emulat-
ed current ramp. Peak current mode control provides inherent
line voltage 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 1MHz. An
oscillator synchronization pin allows multiple LM25576Q0
regulators to self synchronize or be synchronized to an ex-
ternal clock. The output voltage can be set as low as 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
LM25576Q0 are shown in Figure 1. The LM25576Q0 can be
applied in numerous applications to efficiently step-down a
high, unregulated input voltage. The device is well suited for
telecom, industrial and automotive power bus voltage ranges.
High Voltage Start-Up Regulator
The LM25576Q0 contains a dual-mode internal high voltage
startup regulator that provides the Vcc bias supply for the
FIGURE 2. Vin and Vcc Sequencing
9
PWM controller and boot-strap MOSFET gate driver. The in-
put pin (VIN) can be connected directly to the input voltage,
as high as 42 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 6V to 42V is achieved through
the use of this dual mode regulator.
The output of the Vcc regulator is current limited to 25mA.
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 5.35V 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 5.0V 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 45V. 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.
30071204
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