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LM3485MMX 数据表(PDF) 10 Page - Texas Instruments

部件名 LM3485MMX
功能描述  Hysteretic PFET Buck Controller
Download  23 Pages
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制造商  TI [Texas Instruments]
网页  http://www.ti.com
标志 TI - Texas Instruments

LM3485MMX 数据表(HTML) 10 Page - Texas Instruments

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INPUT VOLTAGE - OUTPUT VOLTAGE (V)
0
20
40
60
80
100
120
140
0
5
10
15
20
25
30
35
L=10PH
L=4.7PH
L=22PH
LM3485
SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013
www.ti.com
Figure 3. Propagation Delay
The operating frequency and output ripple voltage can also be significantly influenced by the speed up capacitor
(Cff). Cff is connected in parallel with the high side feedback resistor, R1. The location of this capacitor is similar
to where a feed forward capacitor would be located in a PWM control scheme. However it's effect on hysteretic
operation is much different. The output ripple causes a current to be sourced or sunk through this capacitor. This
current is essentially a square wave. Since the input to the feedback pin, FB, is a high impedance node, the
current flows through R2. The end result is a reduction in output ripple and an increase in operating frequency.
When adding Cff, calculate the formula above with
α = 1. The value of Cff depend on the desired operating
frequency and the value of R2. A good starting point is 470pF ceramic at 100kHz decreasing linearly with
increased operating frequency. Also note that as the output voltage is programmed below 2.5V, the effect of Cff
will decrease significantly.
CURRENT LIMIT OPERATION
The LM3485 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the PFET or across an
additional sense resistor. When current limit is activated, the LM3485 turns off the external PFET for a period of
9µs(typical). The current limit is adjusted by an external resistor, RADJ.
The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive
input of the ISENSE comparator is the ADJ pin. An internal 5.5µA current sink creates a voltage across the
external RADJ resistor. This voltage is compared to the voltage across the PFET or sense resistor. The ADJ
voltage can be calculated as follows:
VADJ = VIN − (RADJ * 3.0µA)
(5)
Where 3.0µA is the minimum ICL-ADJ value.
The negative input of the ISENSE comparator is the ISENSE pin that should be connected to the drain of the
external PFET. The inductor current is determined by sensing the VDS. It can be calculated as follows.
VISENSE = VIN − (RDSON * IIND_PEAK) = VIN − VDS
(6)
10
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Product Folder Links: LM3485


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