lm25088mhx-2 National Semiconductor Corporation, lm25088mhx-2 Datasheet - Page 15

• Manufacturer Part Number: lm25088mhx-2
• Description: Wide Input Range Non-synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

Soft-Start
The soft-start (SS) feature forces the output to rise linearly
until it reaches the steady-state operating voltage set by the
feedback resistors. The LM25088 will regulate the FB pin to
the SS pin voltage or the internal 1.205V reference, which
ever is lower. At the beginning of the soft-start sequence VSS
= 0V and, an internal 11 µA current source gradually increas-
es the voltage of the external soft-start capacitor (C
internal amplifier clamps the SS pin voltage at 120 mV above
the FB voltage. This feature provides soft-start controlled re-
covery with reduced output overshoot in the event that the
output voltage momentarily dips out of regulation.
HG Output
The LM25088 provides a high current, high-side driver and
associated level shift circuit to drive an external N-Channel
MOSFET. The gate driver works in conjunction with an inter-
nal diode and external bootstrap capacitor. A ceramic boot-
strap capacitor is recommended, and should be connected
directly between the BOOT and SW pins. During the off-time
of the buck switch, the bootstrap capacitor charges from VCC
through an internal diode. When operating with a high PWM
duty cycle, the HG output will be forced-off each cycle for 365
ns (max) to ensure that BOOT capacitor is recharged. A “pre-
charge” circuit, comprised of a MOSFET between SW and
GND, is turned ON during the forced off-time to help replenish
the BOOT capacitor. The pre-charge circuit provides charge
to the BOOT capacitor under light load or pre-biased load
conditions when the SW voltage does not remain low during
the entire off-time.
Thermal Protection
Internal thermal shutdown circuitry is provided to protect the
integrated circuit in the event the maximum operating tem-
perature 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 supply of the controller. The feature
prevents catastrophic failures from accidental device over-
heating.
FIGURE 8. Current Limit Restart Timing Diagram
SS
). An
15
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. The Bill of Mate-
rials for this design is listed in Table 1.The circuit shown in
Figure 14 and 15 is configured for the following specifications:
•
•
•
•
TIMING RESISTOR
The RT resistor sets the oscillator switching frequency. High-
er frequencies result in smaller size components such as the
inductor and filter capacitors. However, operating at higher
frequencies also results in higher MOSFET and diode switch-
ing losses. Operation at 250 kHz was selected for this exam-
ple as a reasonable compromise between size and efficiency.
The value of RT resistor can be calculated as follows:
The nearest standard value of 24.9Ω was chosen for RT.
OUTPUT INDUCTOR
The inductor value is determined based on the operating fre-
quency, load current, ripple current and the input and output
voltages.
Knowing the switching frequency (f
rent (I
output voltage (V
follows:
Output Voltage = 5V
Input Voltage = 5.5V to 36V
Maximum Load Current = 7A
Switching Frequency = 250 kHz
PP
), maximum input voltage (VIN
OUT
), the inductor value can be calculated as
SW
30087617
), maximum ripple cur-
(max)
) and the nominal
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