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LM34914 Folha de dados(PDF) 11 Page - Texas Instruments

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Nome de Peças LM34914
Descrição Electrónicos  Ultra Small 1.25A Step-Down Switching Regulator with Intelligent Current Limit
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Fabricante Electrônico  TI1 [Texas Instruments]
Página de início  http://www.ti.com
Logo TI1 - Texas Instruments

LM34914 Folha de dados(HTML) 11 Page - Texas Instruments

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R3(min) =
25 mV x (R1 + R2)
R2 x IOR (min)
IOR (min) =
VOUT x (VIN (min) - VOUT)
L1 x FS x VIN (min)
L1 =
VOUT x (VIN (max) - VOUT)
IOR (max) x FS x VIN (max)
RON t
100 ns x (VIN(MAX) ± 1.5V)
1.15 x 10
-10
- 1.4 k:
LM34914
www.ti.com
SNVS453A – MAY 2007 – REVISED DECEMBER 2007
- Minimum and maximum load current (IOUT(min) and IOUT(max))
- Switching Frequency (FS)
R1 and R2: These resistors set the output voltage. The ratio of these resistors is calculated from:
R1/R2 = (VOUT/2.5V) - 1
(7)
R1 and R2 should be chosen from standard value resistors in the range of 1.0 k
Ω - 10 kΩ which satisfy the
above ratio.
RON: The resistor sets the on-time, and consequently, the switching frequency. Its value can be determined using
equation 5 based on the frequency, or equation 4 if a specific on-time is required. The minimum allowed value for
RON is calculated from:
(8)
L1: The main parameter affected by the inductor is the output current ripple amplitude (IOR). The minimum load
current is used to determine the maximum allowable ripple. In order to maintain continuous conduction mode the
valley should not reach 0 mA. This is not a requirement of the LM34914, but serves as a guideline for selecting
L1. For this case, the maximum ripple current is:
IOR(MAX) = 2 x IOUT(min)
(9)
If the minimum load current is zero, use 20% of IOUT(max) for IOUT(min) in equation 6. The ripple calculated in
Equation 6 is then used in the following equation:
(10)
where Fs is the switching frequency. This provides a minimum value for L1. The next larger standard value
should be used, and L1 should be rated for the peak current level, equal to IOUT(max) + IOR(max)/2.
C2 and R3: Since the LM34914 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the
required ripple at VOUT is increased by R1 and R2. This necessary ripple is created by the inductor ripple current
flowing through R3, and to a lesser extent by C2 and its ESR. The minimum inductor ripple current is calculated
using equation 7, rearranged to solve for IOR at minimum VIN.
(11)
The minimum value for R3 is then equal to:
(12)
Typically R3 is less than 5
Ω. C2 should generally be no smaller than 3.3 µF, although that is dependent on the
frequency and the desired output characteristics. C2 should be a low ESR good quality ceramic capacitor.
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: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed
transitions at the SW pin may inadvertently affect the IC’s operation through external or internal EMI. The diode
should be rated for the maximum input voltage (VIN(max)), the maximum load current (IOUT(max)), and the peak
current which occurs when the current limit and maximum ripple current are reached simultaneously. The diode’s
average power dissipation is calculated from:
PD1 = VF x IOUT x (1-D)
(13)
where VF is the diode's forward voltage drop, and D is the duty cycle.
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 dynamic impedance is high (effectively a current source), it supplies the average input current, but not
the ripple current.
Copyright © 2007, Texas Instruments Incorporated
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