LM25115MT/NOPB National Semiconductor, LM25115MT/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM25115MT/NOPB
• Description: IC CTRLR SSPR 42V LD REG 16TSSOP
• Manufacturer: National Semiconductor

Specifications of LM25115MT/NOPB

• Pwm Type: Voltage/Current Mode
• Number Of Outputs: 2
• Frequency - Max: 1MHz
• Duty Cycle: 90%
• Voltage - Supply: 5 V ~ 7.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 16-TSSOP
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM25115MT; *LM25115MT/NOPB; LM25115MT

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Synchronization (SYNC) and Feed-forward (RAMP)
The recommended SYNC input current range is 50µA to
150µA. The SYNC pin resistor (R
set the SYNC current (I
phase signal amplitude, V
that I
change in phase signal amplitude. The SYNC pin resistor is
therefore:
Once I
RAMP signal amplitude may be programmed by selecting
the proper RAMP pin capacitor value. The recommended
peak amplitude of the RAMP waveform is 1V to 1.75V. The
C
amplitude with the nominal phase signal voltage and pulse
width.
Where
C
I
T
V
For example,
Main channel output = 3.3V. Phase signal maximum ampli-
tude = 12V. Phase signal frequency = 250kHz
• Set I
• T
• Assume desired V
• C
• C
SYNC
ON
RAMP
RAMP
RAMP
tude (12V):
I
(R
R
(3.3V/12V) / 250kHz = 1.1µs
/ 1.5V
SYNC
= corresponding phase signal pulse width
ON
SYNC
RAMP
RAMP
SYNC
SYNC
= SYNC pin current current
SYNC
= RAMP pin capacitance
= desired RAMP amplitude (1V to 1.75V)
capacitor is chosen to provide the desired RAMP
SYNC
= Main channel duty cycle / Phase frequency =
= 150µA = V
R
= 12V/150µA - 2.5k
= (3 x I
= 330pF
stays within the recommended range over a 3:1
+ 2.5 k )
C
SYNC
has been established by selecting R
RAMP
= 150µA with phase signal at maximum ampli-
= (V
SYNC
= (3 x I
PHASE(max)
RAMP
PHASE(max)
x T
SYNC
ON
SYNC
PHASE(max)
= 1.5V
) / V
) to 150µA with the maximum
x T
SYNC
= 77.5k
/ 150µA) - 2.5k
RAMP
/ (R
ON
SYNC
) should be selected to
. This will guarantee
) / V
= (3 x 150µA x 1.1µs)
FIGURE 3. Line Feed-forward Waveforms
+ 2.5 k ) = 12V /
RAMP
SYNC
, the
10
Error Amplifier and Soft-Start (FB,
CO, & COMP, SS)
An internal wide bandwidth error amplifier is provided within
the LM25115 for voltage feedback to the PWM controller.
The amplifier’s inverting input is connected to the FB pin.
The output of the auxiliary converter is regulated by connect-
ing a voltage setting resistor divider between the output and
the FB pin. Loop compensation networks are connected
between the FB pin and the error amplifier output (COMP).
The amplifier’s non-inverting input is internally connected to
the SS pin. The SS pin is biased at 0.75V by a resistor
divider connected to the internal 1.27V bandgap reference.
When the VCC voltage is below the UVLO threshold, the SS
pin is discharged to ground. When VCC rises and exceeds
the positive going UVLO threshold (4.25V), the SS pin is
released and allowed to rise. If an external capacitor is
connected to the SS pin, it will be charged by the internal
resistor divider to gradually increase the non-inverting input
of the error amplifier to 0.75V. The equivalent impedance of
the SS resistor divider is nominally 60k
the charging time constant of the SS capacitor. During start-
up, the output of the LM25115 converter will follow the
exponential equation:
Where
Rss = internal resistance of SS pin (60k )
Css = external Soft-Start capacitor
VOUT(final) = regulator output set point
The initial v / t of the output voltage is VOUT(final) / Rss x
Css and VOUT will be within 1% of the final regulation level
after 4.6 time constants or when t = 4.6 x Rss x Css.
Pull-up current for the error amplifier output is provided by an
internal 300µA current source. The PWM threshold signal at
the COMP pin can be controlled by either the open drain
error amplifier or the open drain current amplifier connected
through the CO pin to COMP. Since the internal error ampli-
fier is configured as an open drain output it can be disabled
by connecting FB to ground. The current sense amplifier and
current limiting function will be described in a later section.
VOUT(t) = VOUT(final) x (1 - exp(-t/R
(Continued)
20172613
which determines
SS
x C
SS
))