Texas Instruments LM22670 Inverting Output Regulator LM22670INVEVAL/NOPB LM22670INVEVAL/NOPB 데이터 시트
제품 코드
LM22670INVEVAL/NOPB
C
OUTMIN
=
I
OUT
x D
F x
'
V
OUT
ESR =
'
V
OUT
I
PEAK
Output Capacitor Selection
9
Output Capacitor Selection
The output capacitor needs to be selected primarily for a low ESR value, but the capacitance must also be
able to deliver the maximum load current when the switch is on. The ESR value determines the load
impedance and output voltage ripple at the first moment diode, D1, becomes forward biased. Thus, the
required ESR for a desired output voltage ripple,
able to deliver the maximum load current when the switch is on. The ESR value determines the load
impedance and output voltage ripple at the first moment diode, D1, becomes forward biased. Thus, the
required ESR for a desired output voltage ripple,
Δ
V
OUT
, is calculated as shown in
:
(11)
The minimum output capacitor value, C
OUTMIN
, for a desired output voltage ripple and load current is:
(12)
10
Input Capacitor Selection
The input capacitor needs to be selected based on its low ESR value and the high RMS current rating
capable of supporting high current changes on the input of the application. Low ESR bypass capacitors
located close to the input pin of the switching regulator are recommended. A larger ESR input capacitor is
useful for input filtering purposes to reduce inductive kicks on the supply line and to keep the input filter
corner frequencies away from the bandwidth of the switching regulator.
capable of supporting high current changes on the input of the application. Low ESR bypass capacitors
located close to the input pin of the switching regulator are recommended. A larger ESR input capacitor is
useful for input filtering purposes to reduce inductive kicks on the supply line and to keep the input filter
corner frequencies away from the bandwidth of the switching regulator.
In general, applications using the polarity-inverting (buck-boost) topology generate noise on both the input
as well as the output. This noise makes the input and the output capacitors important components.
as well as the output. This noise makes the input and the output capacitors important components.
11
Synchronization and Adjustable Frequency
To use the synchronization feature, it is important to apply a synchronization voltage in reference to the
LM22670 ground pin, GND, that has the same potential as the negative output voltage in an inverting
topology. Some level shifting of the synchronization pulse might be necessary to stay within the absolute
maximum rating of the RT/SYNC pin.
LM22670 ground pin, GND, that has the same potential as the negative output voltage in an inverting
topology. Some level shifting of the synchronization pulse might be necessary to stay within the absolute
maximum rating of the RT/SYNC pin.
The switching frequency can be adjusted higher or lower than 500 kHz by connecting a resistor from the
RT/SYNC pin to the LM22670 GND pin. for more details about the synchronizing and adjustable
frequency features, see the LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER®, Step-Down Voltage
Regulator with Features Data Sheet (
RT/SYNC pin to the LM22670 GND pin. for more details about the synchronizing and adjustable
frequency features, see the LM22670/LM22670Q 42V, 3A SIMPLE SWITCHER®, Step-Down Voltage
Regulator with Features Data Sheet (
).
12
Precision Enable
The LM22670 can be shut down if the EN pin is pulled low. In the inverting topology, this means that the
EN pin is pulled to a voltage close to the GND pin voltage, which is the negative output voltage. If an
external signal is applied, care must be taken so that the voltage at the EN pin is never higher than the
maximum allowed voltage according to the absolute maximum rating in the LM22670 datasheet in
reference to the GND pin. Since the GND pin of the LM22670 becomes the negative output voltage in an
inverting application, level shifting might be necessary when using the EN pin. If the EN pin is not used in
an application, it may be left floating.
EN pin is pulled to a voltage close to the GND pin voltage, which is the negative output voltage. If an
external signal is applied, care must be taken so that the voltage at the EN pin is never higher than the
maximum allowed voltage according to the absolute maximum rating in the LM22670 datasheet in
reference to the GND pin. Since the GND pin of the LM22670 becomes the negative output voltage in an
inverting application, level shifting might be necessary when using the EN pin. If the EN pin is not used in
an application, it may be left floating.
13
PCB Layout Guidelines
The printed circuit board (PCB) layout for the LM22670 switching regulator in polarity-inverting topology is
shown in
shown in
. Similar PCB layouts can be used for other versions of the LM2267X SIMPLE
SWITCHER family. It is very important to place the input capacitor as close as possible to the input pin of
the switching regulator. In order to achieve optimal performance, the switching regulator needs to be
properly grounded. It is recommended to use a separate ground plane and a single point ground structure.
Especially, at load currents above 1A, trace layout and component placement is critical, otherwise, high
switching currents will cause malfunction. The parasitic trace inductance is often the main cause of high
voltage spikes as well as EMI problems on the input and output lines.
the switching regulator. In order to achieve optimal performance, the switching regulator needs to be
properly grounded. It is recommended to use a separate ground plane and a single point ground structure.
Especially, at load currents above 1A, trace layout and component placement is critical, otherwise, high
switching currents will cause malfunction. The parasitic trace inductance is often the main cause of high
voltage spikes as well as EMI problems on the input and output lines.
5
SNVA363E – September 2008 – Revised April 2013
AN-1888 LM22670 Evaluation Board Inverting Topology
Copyright © 2008–2013, Texas Instruments Incorporated