RT =

[(1 / 300 x 10

3) ± 580 x 10-9]

135 x 10

-12

LM5576Q0

www.ti.com

SNOSB24B – OCTOBER 2008 – REVISED APRIL 2013

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 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.

THERMAL PROTECTION

Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum junction

temperature 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 overheating.

APPLICATION INFORMATION

EXTERNAL COMPONENTS

The procedure for calculating the external components is illustrated with the following design example. The

circuit shown in Figure 9 is configured for the following specifications:

•

VOUT = 5V

•

VIN = 7V to 75V

•

Fs = 300kHz

•

Minimum load current (for CCM) = 250mA

•

Maximum load current = 3A

R3 (RT)

RT sets the oscillator switching frequency. Generally, higher frequency applications are smaller but have higher

losses. Operation at 300kHz was selected for this example as a reasonable compromise for both small size and

high efficiency. The value of RT for 300kHz switching frequency can be calculated as follows:

(7)

The nearest standard value of 21k

Ω was chosen for RT.

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