Texas Instruments LM22670 Inverting Output Regulator LM22670INVEVAL/NOPB LM22670INVEVAL/NOPB 데이터 시트
제품 코드
LM22670INVEVAL/NOPB
LM22670-ADJ
+
VIN
EN
SW
BOOT
FB
GND
VIN 6V to 35V
VOUT -5V
C1
C2
R1
R2
L1
D1
C3
C4
C5*
C7
VOUT
RT/SYNC
2.2
éF
22
éF
4.7
éF
60V 5A
120
éF
10
éH
7.32 k
Ö
2.55 k
Ö
10 nF
+
* component not populated on LM22670INVEVAL evaluation board
C6
4.7
4.7
éF
C8
2.2
éF
Principle of Operation
3
Principle of Operation
The polarity-inverting converter, shown in
, uses the basic principle of energy storage in the
inductor, L1, during on-time and transfers the energy through the diode, D1, to the output during off-time.
When the switch turns on, the diode is reverse biased and the inductor current will ramp up linearly. When
the switch turns off, the inductor will reverse its polarity in order to maintain the peak switch current. At
that time, the diode, D1, will be forward biased and the energy stored in the inductor will be transferred to
the load as well as the output capacitor, C4.
When the switch turns on, the diode is reverse biased and the inductor current will ramp up linearly. When
the switch turns off, the inductor will reverse its polarity in order to maintain the peak switch current. At
that time, the diode, D1, will be forward biased and the energy stored in the inductor will be transferred to
the load as well as the output capacitor, C4.
Since the switch node is negative with respect to ground, the output voltage across the output capacitors
(C4 and C5) will become negative.
(C4 and C5) will become negative.
This type of polarity-inverting converter can step-up and step-down the magnitude of the input voltage,
which makes this circuit a buck-boost converter. However, the output voltage is always negative in
reference to ground.
which makes this circuit a buck-boost converter. However, the output voltage is always negative in
reference to ground.
Figure 1. Evaluation Board Schematic Inverting Topology
4
Design Considerations
shows the typical configuration of a polarity-inverting converter using the LM22670 switching
regulator. This inverting topology design can be implemented with any member of the LM2267X SIMPLE
SWITCHER
SWITCHER
®
family. Note that the ground pin (GND) of the LM22670 is connected to the negative output,
V
OUT
, and the feedback resistor divider is referred to GND. No extra level shift and inversion of the
feedback signal is required to regulate the negative output voltage. This buck-boost application is also
possible with the fixed voltage version of the LM22670 by connecting the feedback pin directly to ground
of the system. A polarity-inverting topology is particularly difficult to stabilize as it has a right-half plane
zero in its control to output transfer function. Two compensation capacitor, C6 and C7, are connected from
the input to the negative output in order to provide more phase margin and stabilize the loop. For output
currents less than 100 mA, the converter can be operated in discontinuous current conduction mode
(DCM) and capacitors C6 and C7 are not required. When capacitors C6 and C7 are used and voltage is
first applied to the application, the initial capacitor charge current causes a positive voltage spike on the
output. This positive voltage spike is typically too small to cause any damage on the output capacitor. The
initial input capacitor charge current will cause a voltage drop across the capacitor ESR. Since the ESR
from capacitors C6 and C7 and output capacitors C4 and C5 form a voltage divider, the magnitude of the
initial voltage spike will be dependent upon the ESR values of these capacitors. Since the overall output
capacitor ESR value is typically larger than the compensation capacitor ESR value, the initial voltage
spike will be typically below 500 mV. The faster the input voltage slew rate applied to the circuit, the larger
the positive voltage spike. If the inductor DC resistance is 2
possible with the fixed voltage version of the LM22670 by connecting the feedback pin directly to ground
of the system. A polarity-inverting topology is particularly difficult to stabilize as it has a right-half plane
zero in its control to output transfer function. Two compensation capacitor, C6 and C7, are connected from
the input to the negative output in order to provide more phase margin and stabilize the loop. For output
currents less than 100 mA, the converter can be operated in discontinuous current conduction mode
(DCM) and capacitors C6 and C7 are not required. When capacitors C6 and C7 are used and voltage is
first applied to the application, the initial capacitor charge current causes a positive voltage spike on the
output. This positive voltage spike is typically too small to cause any damage on the output capacitor. The
initial input capacitor charge current will cause a voltage drop across the capacitor ESR. Since the ESR
from capacitors C6 and C7 and output capacitors C4 and C5 form a voltage divider, the magnitude of the
initial voltage spike will be dependent upon the ESR values of these capacitors. Since the overall output
capacitor ESR value is typically larger than the compensation capacitor ESR value, the initial voltage
spike will be typically below 500 mV. The faster the input voltage slew rate applied to the circuit, the larger
the positive voltage spike. If the inductor DC resistance is 2
Ω
or greater and the initial start-up current is
high, the positive voltage spike may be higher than 500 mV. An additional clamping diode, D2, can be
used in parallel to the output capacitor C4 to clamp this positive voltage spike to typically 300 mV if a
small Schottky diode is used. Shown in
used in parallel to the output capacitor C4 to clamp this positive voltage spike to typically 300 mV if a
small Schottky diode is used. Shown in
. In most cases this clamp is not required.
2
AN-1888 LM22670 Evaluation Board Inverting Topology
SNVA363E – September 2008 – Revised April 2013
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