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AAT1120 Schematic ( PDF Datasheet ) - AAT

Teilenummer AAT1120
Beschreibung Step-Down Converter
Hersteller AAT
Logo AAT Logo 




Gesamt 20 Seiten
AAT1120 Datasheet, Funktion
AAT1120
500mA Step-Down Converter
General Description
Features
SwitchReg
The AAT1120 SwitchReg is a 1.5MHz step-down
converter with an input voltage range of 2.7V to
5.5V and output as low as 0.6V. Its low supply
current, small size, and high switching frequency
make the AAT1120 the ideal choice for portable
applications.
The AAT1120 delivers up to 500mA of load current,
while maintaining a low 30μA no load quiescent cur-
rent. The 1.5MHz switching frequency minimizes
the size of external components, while keeping
switching losses low. The AAT1120 feedback and
control delivers excellent load regulation and tran-
sient response with a small output inductor and
capacitor.
The AAT1120 is available in a Pb-free, 8-pin, 2x2mm
STDFN package and is rated over the -40°C to
+85°C temperature range.
• VIN Range: 2.7V to 5.5V
• VOUT Range: 0.6V to VIN
• Up to 500mA Output Current
• Up to 96% Efficiency
• 30μA Typical Quiescent Current
• 1.5MHz Switching Frequency
• Soft-Start Control
• Over-Temperature and Current Limit
Protection
• 100% Duty Cycle Low-Dropout Operation
• <1μA Shutdown Current
• Small External Components
• Ultra-Small STDFN22-8 Package
• Temperature Range: -40°C to +85°C
Applications
www.DataSheet4U.com
Bluetooth® Headsets
Cellular Phones
Digital Cameras
Handheld Instruments
Micro Hard Disk Drive
Portable Music Players
USB Devices
Typical Application
VIN
C1
4.7µF
AAT1120
VP LX
VIN
EN FB
GND PGND
VO = 1.8V
L1
3.0μH
R1
118kΩ
500mA
R2
59kΩ
C2
4.7µF
1120.2007.01.1.0
1






AAT1120 Datasheet, Funktion
Typical Characteristics
Soft Start
(VIN = 3.6V; VOUT = 1.8V; 500mA)
5.0
4.0
3.0 VEN
2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
VO
I LX
Time (100µs/div)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
AAT1120
500mA Step-Down Converter
0.30
0.20
0.10
0.00
-0.10
-0.20
-0.30
2.5
Line Regulation
(VOUT = 1.8V)
IOUT = 50mA
IOUT = 10mA
IOUT = 250mA
IOUT = 0mA
IOUT = 150mA
3.0 3.5 4.0 4.5 5.0 5.5 6.0
Input Voltage (V)
Output Voltage Error vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 500mA)
3.0
2.0
1.0
0.0
-1.0
-2.0
-3.0
-40
-20
0
20 40 60 80 100
Temperature (°C)
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
-4.0
-6.0
-8.0
-10.0
-40
Switching Frequency Variation
vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
-20 0
20 40 60 80
Temperature (°C)
100
Frequency Variation vs. Input Voltage
2.0
1.0 VOUT = 1.8V
0.0
-1.0
-2.0
-3.0 VOUT = 3.0V
-4.0
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
No Load Quiescent Current vs. Input Voltage
50
45
40
35 85°C
30 25°C
25 -40°C
20
15
10
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
6 1120.2007.01.1.0

6 Page









AAT1120 pdf, datenblatt
In applications where the input power source lead
inductance cannot be reduced to a level that does
not affect the converter performance, a high ESR
tantalum or aluminum electrolytic should be placed
in parallel with the low ESR, ESL bypass ceramic.
This dampens the high Q network and stabilizes
the system.
Output Capacitor
The output capacitor limits the output ripple and
provides holdup during large load transitions. A
4.7μF to 10μF X5R or X7R ceramic capacitor typi-
cally provides sufficient bulk capacitance to stabi-
lize the output during large load transitions and has
the ESR and ESL characteristics necessary for low
output ripple. For enhanced transient response
and low temperature operation application, a 10μF
(X5R, X7R) ceramic capacitor is recommended to
stabilize extreme pulsed load conditions.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic out-
put capacitor. During a step increase in load cur-
rent, the ceramic output capacitor alone supplies
the load current until the loop responds. Within two
or three switching cycles, the loop responds and
the inductor current increases to match the load
current demand. The relationship of the output
voltage droop during the three switching cycles to
the output capacitance can be estimated by:
COUT
=
3 · ΔILOAD
VDROOP · FS
Once the average inductor current increases to the
DC load level, the output voltage recovers. The
above equation establishes a limit on the minimum
value for the output capacitor with respect to load
transients.
The internal voltage loop compensation also limits
the minimum output capacitor value to 4.7μF. This
is due to its effect on the loop crossover frequency
(bandwidth), phase margin, and gain margin.
Increased output capacitance will reduce the
crossover frequency with greater phase margin.
12
AAT1120
500mA Step-Down Converter
The maximum output capacitor RMS ripple current
is given by:
I =RMS(MAX)
1
·
3
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
Dissipation due to the RMS current in the ceramic
output capacitor ESR is typically minimal, resulting in
less than a few degrees rise in hot-spot temperature.
Adjustable Output Resistor Selection
Resistors R1 and R2 of Figure 1 program the output
to regulate at a voltage higher than 0.6V. To limit the
bias current required for the external feedback resis-
tor string while maintaining good noise immunity, the
suggested value for R2 is 59kΩ. Decreased resistor
values are necessary to maintain noise immunity on
the FB pin, resulting in increased quiescent current.
Table 2 summarizes the resistor values for various
output voltages.
R1
=
VOUT
VREF
-1⎞⎠
·
R2
=
3.3V
0.6V
-
1⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor, (C3 in Figure 1), can be added.
VOUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
R2 = 59kΩ
R1 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2 = 221kΩ
R1 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 2: Adjustable Resistor Values For
Step-Down Converter.
1120.2007.01.1.0

12 Page





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