Datasheet

D
MAX
= 1 - f
SW
x 320 x 10
-9
LM25119/25119Q
SNVS680G –AUGUST 2010–REVISED JANUARY 2014 www.ti.com
Maximum Duty Cycle
When operating with a high PWM duty cycle, the buck switch will be forced off each cycle for 320ns to ensure
the boot-strap capacitor is recharged and to allow time to sample and hold the current in the low side MOSFET.
This forced off-time limits the maximum duty cycle of the controller. When designing a regulator with high
switching frequency and high duty cycle requirements, a check should be made of the required maximum duty
cycle (including losses) against the graph shown in Figure 6.
The actual maximum duty cycle will vary with the operating frequency as follows:
(6)
Figure 6. Maximum Duty Cycle vs Switching Frequency
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 VCC bias regulators. This feature is designed to prevent catastrophic failures
from overheating and destroying the device.
Application Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Only the
values for the 3.3 V output are calculated since the procedure is the same for the 1.8 V output. The circuit shown
in Figure 15 is configured for the following specifications:
• CH1 output voltage, V
OUT1
= 3.3 V
• CH2 output voltage, V
OUT2
= 1.8 V
• CH1 maximum load current, I
OUT1
= 8 A
• CH2 maximum load current, I
OUT2
= 8 A
• Minimum input voltage, V
IN(MIN)
= 6 V
• Maximum input voltage, V
IN(MAX)
= 36 V
• Switching frequency, f
SW
= 230 kHz
Some component values were chosen as a compromise between the 3.3 V and 1.8 V outputs to allow identical
components to be used on both outputs. This design can be reconfigured in a dual-channel interleaved
configuration with a single 3.3 V output which requires identical power channels.
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