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PDF AS7620 Data sheet ( Hoja de datos )
| Número de pieza | AS7620 | |
| Descripción | 500mA Hysteretic High Voltage Step-Down Converter | |
| Fabricantes | austriamicrosystems AG | |
| Logotipo |  |
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Datasheet
www.DataSheet4U.com
AS7620
500mA Hysteretic High Voltage Step-Down Converter
with Dual Power Monitor
1 General Description
2 Key Features
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. Block Diagram
! 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
Level
Shifter
Hysteretic
Controller
Soft-Start
+
-
VREF
+
-
AS7620
+
-
ISENSE
LX
ILIM
PG
PF
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Revision 1.13
1 - 16
1 page  
AS7620
Datasheet - Typical Operating Characteristics
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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)
Figure 5. GND Current vs. IOUT @ TAMB
1000
10000
VIN=24V
1000
VIN=12V
100 VIN=32V
VIN=5V
10
1
1.0
10.0
100.0
1000.0
Iout (mA)
Figure 7. Current Limit Threshold vs VIN
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
5
ILIM VOUT
10
ILIM FLOATING
ILIM GND
15 20
Vin (V)
25
30
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 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)
Figure 8. Average Switching Frequency vs VIN
1000000
IOUT=500mA
100000
IOUT=50mA
10000
IOUT=5mA
1000
IOUT=500µA
100
5 7 9 11 13 15 1719 21 23 25 27 29 31
Vin (V)
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Revision 1.13
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5 Page 
AS7620
Datasheet - Application Information
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9 Application Information
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.
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
+
Δ----I--L----2-
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.
Free Wheeling Diode
The inductor current is forced through the diode during the off-times. The average current flowing through it is
1-- • fsw • -I--L----I--M-----2----•----L--
2 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.
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.
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.
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Revision 1.13
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