Datasheet

L =
V
OUT
I
PP
x f
SW
V
OUT
1 -
V
IN(max)
x
R
RT
=
152 pF
250 kHz
1
- 280 ns
= 24.5 k:
LM25088
LM25088-Q1
SNVS609H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
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 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. The Bill of
Materials for this design is listed in 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 36V
• 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:
(8)
The nearest standard value of 24.9Ω was chosen for RT.
OUTPUT INDUCTOR
The inductor value is determined based on the operating frequency, load current, ripple current and the input and
output voltages.
Knowing the switching frequency (f
SW
), maximum ripple current (I
PP
), maximum input voltage (VIN
(max)
) and the
nominal output voltage (V
OUT
), the inductor value can be calculated as follows:
(9)
16 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM25088 LM25088-Q1