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PDF AS7620 Data sheet ( Hoja de datos )
| Número de pieza | AS7620 | |
| Descripción | Ultra low Ripple Step Down DC/DC Converter | |
| Fabricantes | ams | |
| Logotipo |  |
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AS7620
650mA, Ultra low Ripple Step Down DC/DC Converter
1 General Description
The AS7620 is an easy-to-use, high-efficiency, high-voltage,
hysteretic step-down DC-DC converter, operating in asynchronous
mode. Its low-power architecture extends hold-up time in battery-
backed and critical applications where maximum up-time over a wide
input supply voltage range is needed, while still providing for high
efficiencies of up to 90% during peak current demands.
Although the AS7620 is optimized for 24V applications found in
industrial and medical systems, its ability to support 100% Duty
Cycle makes the AS7620 ideal for applications demanding maximum
up-time and soft power fail behavior. In combination with low idle
current of only 30µA, on-demand switching reduces operating
current at low load currents.
By selecting an appropriate inductor value, operating current can be
lowered and switching frequencies tuned to certain load conditions.
A pin-strapped current limit input minimizes inductor peak current
and thus inductor size and cost for any given application.
The device further includes output short-circuit protection and
thermal shutdown. In shutdown mode, only 1µA (typ) of current is
consumed.
Figure 1. AS7620 - Block Diagram
2 Key Features
Low quiescent current for efficient partial load operation
Wide Supply Voltage Range, 3.6V to 32V
100% Duty Cycle extends operating range
Pin-programmable cycle-by-cycle current limit
Integrated PMOS eliminates bootstrap capacitor
Resistor-programmable Early Power Fail Warning Input
Power-Good Flag
Thermal Shutdown
Fixed 3.3V and adjustable output (1.2V to VIN)
Small 4x4mm 12-Lead MLPQ Enhanced Power Package
Specified from -40ºC to +125ºC junction and 85ºC maximum
ambient temperatures
3 Applications
The AS7620 is suitable for Industrial 24VDC applications like PLCs,
robotics; Home Security and Building Control applications; Solid-
state utility meters; Signage and LED column power; and Sensor
interfaces.
VIN
SHDN
FB
VEPF
LDO
Temp
Hysteretic
Controller
+
-
VREF
+
-
+
-
Level
Shifter
Soft-Start
AS7620
ISENSE
LX
ILIM
PG
PF
www.ams.com/DC-DC_Step-Up/AS7620
Revision 1.18
1 - 16
1 page  
AS7620
Datasheet - Typical Operating Characteristics
7 Typical Operating Characteristics
Figure 3. Efficiency vs. Output Current, VOUT=4.5V
100
VIN=5V
90
VIN=12V
80 VIN=24V
VIN=32V
70
60
50
40
30
1
10 100
Iout (mA)
1000
Figure 4. PMOS Rdson vs. Temperature
1000
900
800
700
600
500
400
300
200
100
0
-40 -20 0 20 40 60 80 100 120
Temp (C)
Figure 5. GND Current vs. IOUT @ TAMB
10000
VIN=24V
1000
VIN=12V
100 VIN=32V
VIN=5V
10
1
1.0
10.0
100.0
1000.0
Iout (mA)
Figure 6. GND Current vs. Temperature @ IOUT=0A
42.0
40.0
VIN=24V
38.0
36.0
34.0
32.0
30.0
28.0
VIN=32V
VIN=5V
VIN=12V
Temp (C)
www.ams.com/DC-DC_Step-Up/AS7620
Revision 1.18
5 - 16
5 Page 
AS7620
Datasheet - Application Information
9 Application Information
9.1 Input Capacitors
An input capacitor is required to sustain the peak current requested by the turning on of the internal PMOS. When used, the capacitor helps to
reduce the noise and saves the input battery life. The input capacitor has to withstand the input RMS current, which can be calculated by the
following formula:
Irms = IOUT •
V----O----U---T----•----(---V----I-N-----–----V----O---U----T--)-
VIN
(EQ 4)
While designing for wide input/output voltage range, the worst case of Irms=1/2*IOUT must be considered. Suggested capacitors are low ESR
OSCON, polymer, aluminum or MLCCs. Tantalum types are not recommended for their weakness in withstanding big inrush currents.
9.2 Output Inductor
The inductor together with the output capacitor represents the output filter. Using the AS7620, the inductor is charged and completely discharged
at every switching cycle being that the converter is forced to work in DCM. Values from 10µH to 100µH are suitable to work with AS7620 and its
selection should consider the following statements:
Bigger inductor implies lower fsw
Bigger inductor implies lower bandwidth
The inductor must be rated to withstand the peak current (ILIM) and the RMS current Irms =
IOUT2
+
∆----I---L---2-
3
9.3 Output Capacitor
The output capacitor together with the inductor represents the output filter. The bigger the capacitance is, the lower will be the output ripple.
Usually, low ESR MLCCs are preferred as they are inexpensive and small in size. Any value from 10µF is suitable, considering the load transient
specifications of the application as well.
9.4 Free Wheeling Diode
The inductor current is forced through the diode during the off-times. The average current flowing through it is
1--
2
•
fsw
•
-I--L----I--M-----2----•----L--
VOUT
The reverse voltage must be higher than the input voltage and safely it is common to consider 30% more. Normally, a schottky diode is preferred
because of its low forward voltage.
9.5 Stability
Even though the hysteretic voltage mode is intrinsically stable, an excessive noise at the FB could cause instability. For this reason care must be
taken drawing the layout, reducing the noise and shielding the FB path from it. The main noise generator is the switching node, which is
commutated from GND to VIN by the internal PMOS and the free wheeling diode and through which a pulse current flows. It is wise to add a
MLCC capacitor as close as possible to the VIN pin of the device and provide a wide/short path between the LX pin and the external components
(inductor and diode). It is preferable to draw the FB path as far as possible from the LX node and, perhaps, shielding it with a GND track. Another
recommendation is to use low ESL output capacitors, thus avoiding electrolytic parts. A big ESL adds a square wave contribution on the FB that
can make the device work improperly.
9.6 100% Duty Cycle Operation
Thanks to the PMOS structure of the internal switch, the device can actually work at 100% duty cycle. This feature is very helpful during the load
transient, because the maximum power can be transferred to the output in order to recover as fastest. The device will try to work at 100% duty
whenever the FB voltage is lower than the upper window’s threshold. Moreover, in this condition, the safety is always guaranteed by the current
limit.
www.ams.com/DC-DC_Step-Up/AS7620
Revision 1.18
11 - 16
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet AS7620.PDF ] | |
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