Datasheet

R
T
=
[(1 / 300 x 10
3
) ± 580 x 10
-9
]
135 x 10
-12
LM25576, LM25576-Q1
www.ti.com SNVS470G –JANUARY 2007–REVISED APRIL 2013
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 LM25576 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 165°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 Bill of
Materials for this design is listed in Table 1. The circuit shown in Figure 9 is configured for the following
specifications:
• V
OUT
= 5V
• V
IN
= 7V to 42V
• Fs = 300kHz
• Minimum load current (for CCM) = 250mA
• Maximum load current = 3A
R3 (R
T
)
R
T
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 R
T
for 300kHz switching frequency can be calculated as follows:
(5)
The nearest standard value of 21kΩ was chosen for R
T
.
L1
The inductor value is determined based on the operating frequency, load current, ripple current, and the
minimum and maximum input voltage (V
IN(min)
, V
IN(max)
).
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