lm5576q0 National Semiconductor Corporation, lm5576q0 Datasheet - Page 13

• Manufacturer Part Number: lm5576q0
• Description: Simple Switcher 75v, 3a Step-down Switching Regulator
• Manufacturer: National Semiconductor Corporation

Maximum Duty Cycle / Input Drop-
out Voltage
There is a forced off-time of 500ns implemented each cycle
to guarantee sufficient time for the diode current to be sam-
pled. This forced off-time limits the maximum duty cycle of the
buck switch. The maximum duty cycle will vary with the op-
erating frequency.
Where Fs is the oscillator frequency. Limiting the maximum
duty cycle will raise the input dropout voltage. The input
dropout voltage is the lowest input voltage required to main-
tain regulation of the output voltage. An approximation of the
input dropout voltage is:
Where V
Operating at high switching frequency raises the minimum in-
put voltage necessary to maintain regulation.
Current Limit
The LM5576Q0 contains a unique current monitoring scheme
for control and over-current protection. When set correctly,
the emulated current sense signal provides a signal which is
proportional to the buck switch current with a scale factor of
0.5 V / A. The emulated ramp signal is applied to the current
limit comparator. If the emulated ramp signal exceeds 2.1V
(4.2A) the present current cycle is terminated (cycle-by-cycle
current limiting). In applications with small output inductance
and high input voltage the switch current may overshoot due
to the propagation delay of the current limit comparator. If an
overshoot should occur, the diode current sampling circuit will
detect the excess inductor current during the off-time of the
buck switch. If the sample & hold DC level exceeds the 2.1V
current limit threshold, the buck switch will be disabled and
skip pulses until the diode current sampling circuit detects the
inductor current has decayed below the current limit thresh-
old. This approach prevents current runaway conditions due
to propagation delays or inductor saturation since the inductor
current is forced to decay following any current overshoot.
Soft-Start
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
D
is the voltage drop across the re-circulatory diode.
D
MAX
= 1 - Fs x 500ns
13
stresses and surges. The internal soft-start current source,
set to 10µA, gradually increases the voltage of an external
soft-start capacitor connected to the SS pin. The soft-start
capacitor voltage is connected to the reference input of the
error amplifier. Various sequencing and tracking schemes
can be implemented using external circuits that limit or clamp
the voltage level of the SS pin.
In the event a fault is detected (over-temperature, Vcc UVLO,
SD) the soft-start capacitor will be discharged. When the fault
condition is no longer present a new soft-start sequence will
commence.
Boost Pin
The LM5576Q0 integrates an N-Channel buck switch and as-
sociated floating high voltage level shift / gate driver. This gate
driver circuit works in conjunction with an internal diode and
an external bootstrap capacitor. A 0.022µF ceramic capacitor,
connected with short traces between the BST pin and SW pin,
is recommended. During the off-time of the buck switch, the
SW pin voltage is approximately -0.5V and the bootstrap ca-
pacitor is charged from Vcc through the internal bootstrap
diode. When operating with a high PWM duty cycle, the buck
switch will be forced off each cycle for 500ns to ensure that
the bootstrap capacitor is recharged.
Under very light load conditions or when the output voltage is
pre-charged, the SW voltage will not remain low during the
off-time of the buck switch. If the inductor current falls to zero
and the SW pin rises, the bootstrap capacitor will not receive
sufficient voltage to operate the buck switch gate driver. For
these applications, the PRE pin can be connected to the SW
pin to pre-charge the bootstrap capacitor. The internal pre-
charge MOSFET and diode connected between the PRE pin
and PGND turns on each cycle for 265ns just prior to the onset
of a new switching cycle. If the SW pin is at a normal negative
voltage level (continuous conduction mode), then no current
will flow through the pre-charge MOSFET/diode.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction temper-
ature is exceeded. When activated, typically at 180°C, the
controller is forced into a low power reset state, disabling the
output driver and the bias regulator. This feature is provided
to prevent catastrophic failures from accidental device over-
heating.
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