Datasheet

152 pF

250 kHz

- 280 ns

= 24.5 k:

LM5088, LM5088-Q1

SNVS600H –DECEMBER 2008–REVISED MARCH 2013

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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 LM5088 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 increases the voltage of the external soft-start capacitor (C

). An internal amplifier

clamps the SS pin voltage at 120 mV above the FB voltage. This feature provides soft-start controlled recovery

with reduced output overshoot in the event that the output voltage momentarily dips out of regulation.

HG Output

The LM5088 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 internal diode and external bootstrap capacitor.

A ceramic bootstrap 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 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 supply of the controller. The feature prevents catastrophic

failures from accidental device over-heating.

Applications Information

EXTERNAL COMPONENTS

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

Table 1.The circuit shown in Figure 28 and Figure 29 is configured for the following specifications:

• Output Voltage = 5V

• Input Voltage = 5.5V to 55V

• Maximum Load Current = 7A

• Switching Frequency = 250 kHz

TIMING RESISTOR

The RT resistor sets the oscillator switching frequency. Higher 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 switching losses. Operation at 250 kHz was selected for this example as a reasonable

compromise between size and efficiency.

The value of RT resistor can be calculated as follows:

(9)

The nearest standard value of 24.9Ω was chosen for RT.

Product Folder Links: LM5088 LM5088-Q1