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PDF AAT1121 Data sheet ( Hoja de datos )
| Número de pieza | AAT1121 | |
| Descripción | Step-Down Converter | |
| Fabricantes | AAT | |
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AAT1121
1.5MHz, 250mA Step-Down Converter
General Description
The AAT1121 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 AAT1121 the ideal choice for portable
applications.
The AAT1121 delivers 250mA of load current, while
maintaining a low 30µA no load quiescent current.
The 1.5MHz switching frequency minimizes the size
of external components, while keeping switching
losses low. The AAT1121 feedback and control
delivers excellent load regulation and transient
response with a small output inductor and capacitor.
The AAT1121 is available in a Pb-free, 8-pin,
2x2mm TDFN or STDFN package and is rated
over the -40°C to +85°C temperature range.
Features
SwitchReg™
• VIN Range: 2.7V to 5.5V
• VOUT Range: 0.6V to VIN
• 250mA Max 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 TDFN22-8 or STDFN22-8
Package
• Temperature Range: -40°C to +85°C
Applications
•
•www.DataSheet4U.com
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•
•
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Bluetooth™ Headsets
Cellular Phones
Digital Cameras
Handheld Instruments
Portable Music Players
USB Devices
Typical Application
VIN
C1
4.7µF
AAT1121
VP LX
VIN
EN FB
GND PGND
VO = 1.8V
L1
3.0μH
R1
118kΩ
250mA
R2
59kΩ
C2
4.7µF
1121.2007.03.1.2
1
1 page  
Typical Characteristics
Efficiency vs. Load
(VOUT = 1.2V; L = 1.5µH)
100
90 VIN = 2.7V
80 VIN = 3.6V
70
60 VIN = 4.2V
50 VIN = 5.0V
40
30
0.1
1
10 100
Output Current (mA)
AAT1121
1.5MHz, 250mA Step-Down Converter
1000
1.0
0.5
0.0
-0.5
-1.0
0.1
DC Load Regulation
(VOUT = 1.2V; L = 1.5µH)
VIN = 3.6V
VIN = 2.7V VIN = 4.2V
1 10 100
Output Current (mA)
1000
Efficiency vs. Load
(VOUT = 1.8V; L = 3.3µH)
100
VIN = 2.7V
90
VIN = 3.6V
80
70
60 VIN = 4.2V
50
40
0.1
1
10 100
Output Current (mA)
1000
DC Load Regulation
(VOUT = 1.8V; L = 3.3µH)
1.0
0.5
VIN = 3.6V
0.0
VIN = 2.7V
-0.5
VIN = 4.2V
-1.0
0.1
1
10 100
Output Current (mA)
1000
Efficiency vs. Load
(VOUT = 3.0V; L = 4.7µH)
100
90 VIN = 3.6V
80 VIN = 4.2V
70 VIN = 5.0V
60
50
40
0.1
1
10 100
Output Current (mA)
1000
1.0
0.5
0.0
-0.5
-1.0
0.1
DC Load Regulation
(VOUT = 3.0V; L = 4.7µH)
VIN = 4.2V VIN = 3.6V
VIN = 5.0V
1 10 100
Output Current (mA)
1000
1121.2007.03.1.2
5
5 Page 
AAT1121
1.5MHz, 250mA Step-Down Converter
Output Voltage (V)
1.0
1.2
1.5
1.8
2.5
3.0
3.3
L1 (µH)
1.5
2.2
2.7
3.0
3.9
4.7
5.6
Table 1: Inductor Values.
The 3.0µH CDRH2D09 series inductor selected
from Sumida has a 150mΩ DCR and a 470mA DC
current rating. At full load, the inductor DC loss is
9.375mW which gives a 2.08% loss in efficiency for
a 250mA, 1.8V output.
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (VPP) and solve for CIN. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
VO · ⎛1 - VO ⎞
VIN ⎝ VIN ⎠
CIN =
⎛ VPP
⎝ IO
- ESR⎞⎠ · FS
VO
VIN
·
⎛⎝1 -
VO ⎞
VIN ⎠
=
1
4
for
VIN
=
2
×
VO
1
CIN(MIN) = ⎛ VPP
⎝ IO
- ESR⎞⎠ · 4 · FS
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
The maximum input capacitor RMS current is:
IRMS = IO ·
VO · ⎛1 - VO ⎞
VIN ⎝ VIN ⎠
1121.2007.03.1.2
The input capacitor RMS ripple current varies with
the input and output voltage and will always be less
than or equal to half of the total DC load current.
VO · ⎛1 - VO ⎞ = D · (1 - D) = 0.52 = 1
VIN ⎝ VIN ⎠
2
for VIN = 2 x VO
I =RMS(MAX)
IO
2
VO · ⎛1 - VO ⎞
The term VIN ⎝ VIN ⎠ appears in both the input
voltage ripple and input capacitor RMS current
equations and is a maximum when VO is twice VIN.
This is why the input voltage ripple and the input
capacitor RMS current ripple are a maximum at
50% duty cycle.
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the
AAT1121. Low ESR/ESL X7R and X5R ceramic
capacitors are ideal for this function. To minimize
stray inductance, the capacitor should be placed as
closely as possible to the IC. This keeps the high
frequency content of the input current localized,
minimizing EMI and input voltage ripple.
The proper placement of the input capacitor (C1)
can be seen in the evaluation board layout in
Figure 2.
A laboratory test set-up typically consists of two
long wires running from the bench power supply to
the evaluation board input voltage pins. The induc-
tance of these wires, along with the low-ESR
ceramic input capacitor, can create a high Q net-
work that may affect converter performance. This
problem often becomes apparent in the form of
excessive ringing in the output voltage during load
transients. Errors in the loop phase and gain meas-
urements can also result.
Since the inductance of a short PCB trace feeding
the input voltage is significantly lower than the
power leads from the bench power supply, most
applications do not exhibit this problem.
11
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| PDF Descargar | [ Datasheet AAT1121.PDF ] | |
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