1 - Fs x 500 ns

LM5576, LM5576-Q1

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SNVS447I – JANUARY 2007 – REVISED APRIL 2013

Maximum Duty Cycle / Input Drop-out Voltage

There is a forced off-time of 500ns implemented each cycle to ensure sufficient time for the diode current to be

sampled. This forced off-time limits the maximum duty cycle of the buck switch. The maximum duty cycle will

vary with the operating frequency.

where

•

Fs is the oscillator frequency

(5)

Limiting the maximum duty cycle will raise the input dropout voltage. The input dropout voltage is the lowest input

voltage required to maintain regulation of the output voltage. An approximation of the input dropout voltage is:

where

•

(6)

Operating at high switching frequency raises the minimum input voltage necessary to maintain regulation.

Current Limit

The LM5576 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 threshold. 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 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 LM5576 integrates an N-Channel buck switch and associated 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 capacitor 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. For output voltages above 5V, a minimum load

current may still be required to ensure that the SW voltage is pulled low enough to recharge the bootstrap

capacitor.

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