lm25119psqx National Semiconductor Corporation, lm25119psqx Datasheet - Page 15

• Manufacturer Part Number: lm25119psqx
• Description: Lm25119 Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

FIGURE 5. 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 temper-
ature 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 de-
signed to prevent catastrophic failures from overheating and
destroying the device.
Application Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is il-
lustrated with the following design example. Only the values
for the 3.3V output are calculated since the procedure is the
same for the 1.8V output. The circuit shown in
configured for the following specifications:
•
•
•
•
•
•
•
Some component values were chosen as a compromise be-
tween the 3.3V and 1.8V outputs to allow identical compo-
nents to be used on both outputs. This design can be
reconfigured in a dual-channel interleaved configuration with
a single 3.3V output which requires identical power channels.
TIMING RESISTOR
R
Generally, higher frequency applications are smaller but have
higher losses. Operation at 230kHz was selected for this ex-
ample as a reasonable compromise between small size and
high efficiency. The value of R
quency can be calculated as follows:
T
CH1 output voltage, V
CH2 output voltage, V
CH1 maximum load current, I
CH2 maximum load current, I
Minimum input voltage, V
Maximum input voltage, V
Switching frequency, f
sets the switching frequency of each regulator channel.
OUT1
OUT2
SW
IN(MIN)
= 230kHz
IN(MAX)
= 3.3V
= 1.8V
T
OUT1
OUT2
for 230kHz switching fre-
= 6V
= 36V
= 8A
= 8A
30126214
Figure 14
(7)
is
15
A standard value or 22.1kΩ was chosen for R
oscillator frequency is twice the switching frequency and is
about 460kHz.
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, maximum ripple current
(I
(V
The maximum ripple current occurs at the maximum input
voltage. Typically, I
When operating in the diode emulation mode configuration,
the maximum ripple current should be less than twice the
minimum load current. For full synchronous operation, higher
ripple current is acceptable. Higher ripple current allows for a
smaller inductor size, but places more of a burden on the out-
put capacitor to smooth the ripple current. For this example,
a ripple current of 25% of 8A was chosen as a compromise
for the 1.8V output.
The nearest standard value of 6.8μH was chosen for L. Using
the value of 6.8µH for L, calculate I
essary if the chosen value of L differs significantly from the
calculated value.
CURRENT SENSE RESISTOR
Before determining the value of current sense resistor (R
is valuable to understand the K factor, which is the ramp slope
multiple chosen for slope compensation. The K factor can be
varied from 1 to 3 in practice and is defined as:
PP
OUT
), maximum input voltage and the nominal output voltage
), the inductor value can be calculated:
FIGURE 6. Inductor Current
PP
is 20% to 40% of the full load current.
PP
again. This step is nec-
T
. The internal
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30126215
S
(10)
(11)
), it
(8)
(9)