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LM25010SDX Fiches technique(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce LM25010SDX
Description  42V, 1.0A Step-Down Switching Regulator
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM25010SDX Fiches technique(HTML) 11 Page - National Semiconductor (TI)

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Current Limit (Continued)
mode. The load current (I
OCL) is equal to the current limit
threshold plus half the ripple current (
∆I/2). The ripple ampli-
tude (
∆I) is calculated from:
(9)
The current limit threshold can be increased by connecting
an external resistor (R
CL) between SGND and ISEN. RCL
typically is less than 1
Ω, and the calculation of its value is
explained in the Applications Information section. If the cur-
rent limit threshold is increased by adding R
CL, the maximum
continuous load current should not exceed 1.5A, and the
peak current out of the SW pin should not exceed 2A.
N - Channel Buck Switch and
Driver
The LM25010 integrates an N-Channel buck switch and
associated floating high voltage gate driver. The peak cur-
rent through the buck switch should not exceed 2A, and the
load current should not exceed 1.5A. The gate driver circuit
is powered by the external bootstrap capacitor between BST
and SW (C4), which is recharged each off-time from V
CC
through the internal high voltage diode. The minimum off-
time, nominally 260 ns, ensures sufficient time during each
cycle to recharge the bootstrap capacitor. A 0.022 µF ce-
ramic capacitor is recommended for C4.
Soft-Start
The soft-start feature allows the regulator to gradually reach
a steady state operating point, thereby reducing startup
stresses and current surges. At turn-on, while V
CC is below
the under-voltage threshold (t1 in Figure 1), the SS pin is
internally grounded, and V
OUT is held at 0V. When VCC
exceeds the under-voltage threshold (UVLO) an internal
11.5 µA current source charges the external capacitor (C6)
at the SS pin to 2.5V (t2 in Figure 1). The increasing SS
voltage at the non-inverting input of the regulation compara-
tor gradually increases the output voltage from zero to the
desired value. The soft-start feature keeps the load inductor
current from reaching the current limit threshold during
start-up, thereby reducing inrush currents.
An internal switch grounds the SS pin if V
CC is below the
under-voltage lock-out threshold, if a thermal shutdown oc-
curs, or if the circuit is shutdown using the RON/SD pin.
Thermal Shutdown
The LM25010 should be operated below the Maximum Op-
erating Junction Temperature rating. If the junction tempera-
ture increases during a fault or abnormal operating condi-
tion, the internal Thermal Shutdown circuit activates typically
at 175˚C. The Thermal Shutdown circuit reduces power
dissipation by disabling the buck switch and the on-timer,
and grounding the SS pin. This feature helps prevent cata-
strophic failures from accidental device overheating. When
the junction temperature reduces below approximately
155˚C (20˚C typical hysteresis), the SS pin is released and
normal operation resumes.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is
illustrated with a design example. Referring to the Block
Diagram, the circuit is to be configured for the following
specifications:
V
OUT =5V
V
IN =6Vto40V
F
S = 175 kHz
Minimum load current = 200 mA
Maximum load current = 1.0A
Softstart time=5ms.
R1 and R2: These resistors set the output voltage, and their
ratio is calculated from:
R1/R2 = (V
OUT/2.5V) - 1
(10)
R1/R2 calculates to 1.0. The resistors should be chosen
from standard value resistors in the range of 1.0 k
Ω -10kΩ.
A value of 1.0 k
Ω will be used for R1 and for R2.
R
ON,FS: RON can be chosen using Equation 7 to set the
nominal frequency, or from Equation 6 if the on-time at a
particular V
IN is important. A higher frequency generally
means a smaller inductor and capacitors (value, size and
cost), but higher switching losses. A lower frequency means
20172720
FIGURE 4. Inductor Current - Current Limit Operation
www.national.com
11


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