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LM2695MHX 데이터시트(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
부품명 LM2695MHX
상세설명  High Voltage (30V, 1.25A) Step Down Switching Regulator
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제조업체  NSC [National Semiconductor (TI)]
홈페이지  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2695MHX 데이터시트(HTML) 11 Page - National Semiconductor (TI)

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Softstart
The softstart feature allows the converter to gradually reach
a steady state operating point, thereby reducing start-up
stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an internal 12.3 µA
current source charges up the external capacitor at the SS
pin to 2.5V. The ramping voltage at SS (and the non-
inverting input of the regulation comparator) ramps up the
output voltage in a controlled manner.
An internal switch grounds the SS pin if V
CC is below the
under-voltage lockout threshold, if a thermal shutdown oc-
curs, or if the RON/SD pin is grounded.
Thermal Shutdown
The LM2695 should be operated so the junction temperature
does not exceed 125˚C. If the junction temperature in-
creases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175˚C, takes the controller to a low power
reset state by disabling the buck switch and the on-timer,
and grounding the Softstart pin. This feature helps prevent
catastrophic failures from accidental device overheating.
When the junction temperature reduces below 155˚C (typical
hysteresis = 20˚C), the Softstart pin is released and normal
operation resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components.
R1 and R2: The ratio of these resistors is calculated from:
R1/R2 = (V
OUT/2.5V) - 1
R1 and R2 should be chosen from standard value resistors
in the range of 1.0 k
Ω -10kΩ which satisfy the above ratio.
R
ON: The minimum value for RON is calculated from:
Equation 1 can be used to select R
ON if a specific frequency
is desired as long as the above limitation is met.
L1: The main parameter affected by the inductor is the
output current ripple amplitude (I
OR). The limits for IOR must
be determined at both the minimum and maximum nominal
load currents.
a) If the maximum load current is less than the current limit
threshold (1.25A), the minimum load current is used to de-
termine the maximum allowable ripple. To maintain continu-
ous conduction mode the lower peak should not reach 0 mA.
For this case, the maximum ripple current is:
I
OR(MAX1) =2xIO(min)
(7)
The ripple calculated in Equation 7 is then used in the
following equation:
(8)
where V
IN is the maximum input voltage and Fs is deter-
mined from equation 1. This provides a minimum value for
L1. The next larger standard value should be used, and L1
should be rated for the I
PK current level.
b) If the maximum load current is greater than the current
limit threshold (1.25A), the LM2695 ensures the lower peak
reaches 1.25A each cycle, requiring that I
OR be at least twice
the difference. The upper peak, however, must not exceed
2A. For this case, the ripple limits are:
I
OR(MAX2) =2x(2A -IO(max))
(9)
and
I
OR(MIN1) =2x(IO(max) - 1.25A)
(10)
The lesser of Equations 9 and 10 is then used in Equation 8.
If I
OR(MAX2) is used, the maximum VIN is used in Equation 8.
The next larger value should then be used for L1. If I
OR(MIN1)
is used, the minimum V
IN is used in Equation 8. The next
smaller value should then be used for L1. L1 must be rated
for the peak value of the current waveform (I
PK in Figure 7).
C3: The capacitor at the V
CC output provides not only noise
filtering and stability, but also prevents false triggering of the
V
CC UVLO at the buck switch on/off transitions. For this
reason, C3 should be no smaller than 0.1 µF, and should be
a good quality, low ESR, ceramic capacitor.
C2, and R3: Since the LM2695 requires a minimum of 25
mV
p-p of ripple at the FB pin for proper operation, the re-
quired ripple at V
OUT1 is increased by R1 and R2. This
necessary ripple is created by the inductor ripple current
acting on C2’s ESR + R3. The minimum ripple current is
calculated using equation 8, rearranged to solve for I
OR at
minimum V
IN. The minimum ESR for C2 is then equal to:
(11)
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in Figure 3. Generally R3 is less
than 1
Ω. C2 should generally be no smaller than 3.3 µF,
although that is dependent on the frequency and the allow-
able ripple amplitude at V
OUT1. Experimentation is usually
necessary to determine the minimum value for C2, as the
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for C2
than a non-varying load.
D1: The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck
switch is turned on. The forward voltage drop is significant in
the event the output is short-circuited as it is mainly this
diode’s voltage (plus the voltage across the current limit
sense resistor) which forces the inductor current to decrease
during the off-time. For this reason, a higher voltage is better,
although that affects efficiency. A reverse recovery time of
)30 ns, and a forward voltage drop of )0.75V are preferred.
The reverse leakage specification is important as that can
significantly affect efficiency. D1’s reverse voltage rating
must be at least as great as the maximum V
IN, and its
current rating must equal or exceed I
PK Figure 7.
C1 and C5: C1’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at VIN,
on the assumption that the voltage source feeding VIN has
an output impedance greater than zero. If the source’s dy-
namic impedance is high (effectively a current source), it
supplies the average input current, but not the ripple current.
www.national.com
11


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