202037A.pdf

DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
General Description
Features
The AAT2749 is a two channel power regulator that operates from 2.3V to 5.5V. The AAT2749 contains a 3.0MHz,
600mA step-down converter and a low input voltage
300mA LDO regulator.
• VIN Range: 2.3V to 5.5V
• Two Outputs
▪ VOUT_BUCK = 1.8V
▪ VOUT_LDO = 1.0V, 1.2V
• Step-Down Converter
▪ Ultra-Small 0603 Inductor (Height 1mm)
▪ Up to 92% Efficiency
▪ 3.0MHz Switching Frequency
▪ MODE Selection Pin to Select Forced PWM or PWM/
LL Operation Mode
▪ Current Limit Protection
▪ Automatic Soft Start
• LDO Regulator
▪ VINL Range: 1.62V to VIN
▪ 300mA Output Current
▪ Current Limit Protection
• 100μA No Load Quiescent Current
• Over-Temperature Protection
• -40°C to +85°C Temperature Range
The step-down converter can deliver 600mA of load current. The 3.0MHz switching frequency minimizes the size
of external components while keeping switching losses
low. The AAT2749 step-down converter maintains high
efficiency throughout the operating range, which is critical for portable applications. Fixed frequency, low noise
operation can be forced by a logic signal on the MODE
pin.
The AAT2749 also contains a 300mA LDO regulator with
input voltage capability as low as 1.62V. The LDO regulator power input can be tied directly to the output of the
step-down regulator to increase efficiency.
Total quiescent current for the step-down converter and
LDO is a low 100μA under no load condition.
The AAT2749 is available in a Pb-free, space-saving
9-pin chip scale package (CSP) (200μm bump in 400μm
pitch) and is rated over the -40°C to +85°C temperature
range.
Applications
•
•
•
•
•
Battery-Powered Applications
Cellular and Smart Phones
Digital Still and Video Cameras
PDAs, Palmtops
Portable Instruments
Typical Application
2.2μH
(2.3V - 5.5V)
IN_BUCK
CIN_BUCK
4.7μF
(1.62V - 5.5V)
IN_LDO
CIN_LDO
1.0μF
1.8V: 600mA
LX
OUT_BUCK
OUT_LDO
AAT2749
CBUCK
4.7μF
1.2V: 300mA
CLDO
2.2μF
CHIP_ENABLE
MODE
GND
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
1
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Pin Descriptions
Pin #
Symbol
1
2
3
4
5, 9
6
7
IN_LDO
OUT_LDO
OUT_BUCK
MODE
GND
CE
IN_BUCK
8
LX
Description
Input supply voltage for the LDO regulator. May be connected to output of step-down regulator.
LDO power output.
Step-down converter output voltage feedback pin.
Mode pin. To force the buck converter into fixed frequency, PWM mode pull the MODE pin high.
Ground pin.
Chip enable; Logic High enables step-down converter and LDO.
Input supply voltage for the step-down converter.
Switching node. Connect the inductor to this pin. It is internally connected to the drain of both highand low-side MOSFETs.
Pin Configuration
LX
WLCSP-9
(Top View)
GND
9
8
7
IN_BUCK
CE
6
5
4
MODE
1
IN_LDO
GND
3
2
OUT_LDO
OUT_BUCK
2
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Absolute Maximum Ratings1
TA = 25OC unless otherwise noted.
Symbol
VIN_BUCK, VIN_LDO
VLX
VOUT_BUCK
VOUT_LDO
VCE, VMODE
TJ
TLEAD
Description
Value
Input Voltage
LX to GND
OUT_BUCK to GND
OUT_LDO to GND
CE to GND
Maximum Junction Operating temperature
Maximum Soldering Temperature (at leads, 10 sec)
Units
-0.3 to 6.0
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-0.3 to VIN + 0.3
-40 to +150
300
V
O
C
Thermal Information3
Symbol
JA
PD
Description
Value
Thermal Resistance
Maximum Power Dissipation
4
284
352
Units
C/W
mW
O
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions
specified is not implied. Only one Absolute Maximum Rating should be applied at any one time.
2. Based on long-term current density limitation.
3. Mounted on an FR4 board.
4. Derate 20mW/°C above 25°C.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
3
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Electrical Characteristics1
VIN = 3.6V; CIN = COUT_BUCK = 4.7μF; CIN_LDO = 1.0μF; COUT_LDO = 2.2μF; TA = -40oC to 85oC unless otherwise noted.
Typical values are at TA = 25oC.
Symbol
Description
Conditions
Step-Down Converter
VIN_BUCK
Input Voltage
VOUT_BUCK_ACC
Output Voltage Accuracy
VOUT_BUCK_TOL Output Voltage Tolerance
VOUTB/VIN Line Regulation
ILIM
P-Channel Current Limit
High Side Switch On Resistance
RDS(ON)H
RDS(ON)L
Low Side Switch On Resistance
IOUT_BUCK
Out_BUCK Leakage Current
TS
Start-Up Time
FOSC
Oscillator Frequency
LDO Regulator
VIN_LDO
Input Voltage
VOUT_LDO_ACC
Output Voltage Accuracy
VOUT_LDO_TOL Output Voltage Tolerance
VOUT/VIN Line Regulation
ΔVOUT/IOUT Load Regulation
IOUT(MAX)
Maximum Output Current
ILIM
Output Current Limit
VDO
Dropout Voltage
System Quiescent Curent
IQ
ISHDN
Logic
VCE(L)
VCE(H)
ICE
Thermal
TSD
THYS
4
System Quiescent Current
Shutdown Current
Enable Threshold Low
Enable Threshold High
Input Low Current
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
Min
Typ
2.3
VIN = 3.6V, 1mA Load, TA = 25°C, PWM
Mode
0mA to 600mA load, VIN = 2.5V to 5.5V
VIN = 2.5V to 5.5V
591
600
-3.0
Max
Units
5.5
V
609
mV
3.0
%
%/V
mA
mΩ
mΩ
μA
μs
MHz
0.1
1000
360
200
Vout_BUCK = 1.8V
From Enable to Output Regulation
TA = 25°C
VIN_BUCK = 3.6V, VIN_LDO = 1.8V, 1mA Load,
TA = 25°C
0mA to 300mA load; VIN_LDO = 2.5V to 5.5V
VINL = 2.5V to 5.5V
0mA to 300mA load
20
50
3.0
1.62
VIN_BUCK
V
-2
+2
%
+3
%
%/V
%
mA
A
mV
-3
0.6
1.2
300
1
300
VIN_BUCK = 3.6V, VIN_LDO = 1.8V, 200mA load
VOUT_BUCK = 1.8V, No load on LDO, LL Mode
PWM Mode
VCE = GND
100
7
1
0.4
VIN = VOUT = 5.5V
1.4
-1.0
1.0
140
15
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
μA
mA
μA
V
V
μA
°C
°C
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Typical Characteristics
No Load Quiescent Current vs. Input Voltage
(EN_BUCK = EN_LDO = VIN; LL Mode)
(EN_BUCK = EN_LDO = VIN; PWM Mode)
150
14
130
12
110
90
70
T = 85°C
T = 25°C
T = -40°C
50
Input Current (μA)
Input Current (μA)
No Load Quiescent Current vs. Input Voltage
10
8
6
4
T = 85°C
T = 25°C
T = -40°C
2
0
30
2.3
2.8
3.3
3.8
4.3
4.8
5.3
2.3
Input Voltage (V)
3.8
4.3
4.8
5.3
LDO Turn-Off Response Time
(VOUT = 1.2V; VIN = 5V; IOUT = 400mA)
(VOUT = 1.2V; VIN = 5V; IOUT = 400mA)
3.5
6
4
3
4
3
2
2.5
2
2.5
0
2
0
2
-2
1.5
-4
1
1.5
-4
1
-6
0.5
-8
0
-10
3.5
-6
0.5
-8
0
-10
-0.5
Time (50μs/div)
-0.5
Time (10μs/div)
LDO Line Transient Response
LDO Line Transient Response
(VOUT = 1.2V; VIN = 4V to 5V; IOUT = 400mA)
(VOUT = 1.2V; VIN = 4V to 5V; IOUT = 400mA)
1.8
5
1.7
4
1.6
4
1.6
3
1.5
2
1.4
1
1.3
0
1.2
-1
1.1
-2
1
Enable Voltage
(top) (1V/div)
6
1.7
3
1.5
2
1.4
1
1.3
0
1.2
-1
1.1
-2
1
Output Voltage
(bottom) (100mV/div)
1.8
5
Output Voltage
(bottom) (100mV/div)
6
Time (50μs/div)
Output Voltage
(bottom) (500mV/div)
-2
Enable Voltage
(top) (2V/div)
6
Output Voltage
(bottom) (500mV/div)
Enable Voltage
(top) (2V/div)
3.3
Input Voltage (V)
LDO Turn-On Response Time
Enable Voltage
(top) (1V/div)
2.8
Time (50μs/div)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Typical Characteristics
Step-Down Converter Switching Frequency
vs. Input Voltage
Step-Down Converter Switching Frequency
vs. Temperature
Switching Frequency (MHz)
Frequency Variation (%)
(VOUT = 1.8V; IOUT = 100mA)
3
2
1
0
-1
-2
-3
-4
-5
2.3
2.8
3.3
3.8
4.3
4.8
(VOUT = 1.8V; IOUT = 100mA)
3.02
3
2.98
2.96
2.94
2.92
2.9
2.88
2.86
2.84
2.82
5.3
-40
-15
10
Input Voltage (V)
Step-Down Converter Efficiency vs. Load
90
0.8
Efficiency (%)
80
70
60
50
40
VIN = 5.5V
VIN = 4.2V
VIN = 3.3V
VIN = 2.7V
VIN = 2.3V
30
20
10
0
100
Output Error (%)
1
10
VIN = 5.5V
VIN = 4.2V
VIN = 3.3V
VIN = 2.7V
VIN = 2.3V
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
1000
0.1
1
Output Current (mA)
90
0.8
80
0.6
70
60
50
40
VIN = 5.5V
VIN = 4.2V
VIN = 3.3V
VIN = 2.7V
VIN = 2.3V
30
20
10
6
100
1000
Output Error (%)
Efficiency (%)
1
Output Current (mA)
1000
(VOUT = 1.8V; L = 2.2μH; PWM Mode)
100
10
100
Step-Down Converter DC Regulation
(VOUT = 1.8V; L = 2.2μH; PWM Mode)
1
10
Output Current (mA)
Step-Down Converter Efficiency vs. Load
0
0.1
85
(VOUT = 1.8V; L = 2.2μH; LL Mode)
100
1
60
Step-Down Converter DC Regulation
(VOUT = 1.8V; L = 2.2μH; LL Mode)
0.1
35
Temperature (°C)
VIN = 5.5V
VIN = 4.2V
VIN = 3.3V
VIN = 2.7V
VIN = 2.3V
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0.1
1
10
100
Output Current (mA)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
1000
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Step-Down Converter Output Ripple
Step-Down Converter Output Ripple
(VOUT = 1.8V; VIN = 5V; IOUT = 1mA)
(VOUT = 1.8V; VIN = 5V; IOUT = 600mA)
1.84
0.5
1.82
1.8
0.4
1.78
0.3
1.76
0.2
1.74
0.1
1.72
0
1.7
-0.1
1.68
-0.2
Output Voltage
(top) (20mV/div)
0.6
1.82
3.5
3
1.8
2.5
1.78
2
1.76
1.5
1.74
1
1.72
0.5
0
1.7
-0.5
1.68
Time (20μs/div)
Time (200ns/div)
Step-Down Converter N-Channel RDS(ON)
vs. Input Voltage
700
400
600
350
500
300
RDS(ON)L (mΩ)
RDS(ON)H (mΩ)
Step-Down Converter P-Channel RDS(ON)
vs. Input Voltage
400
300
T = 120°C
T = 100°C
T = 85°C
T = 25°C
200
100
0
2.7
3.1
3.5
3.9
4.3
4.7
5.1
250
200
150
50
0
2.7
5.5
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Step-Down Converter Soft Start
Step-Down Converter Line Transient Response
(VOUT = 1.8V; VIN = 5V; IOUT = 600mA)
(VOUT = 1.2V; VIN = 4V to 5V; IOUT = 600mA)
6
2.4
4
3
5
2.3
2
2.5
4
2.2
0
2
3
2.1
-2
1.5
-4
1
-6
0.5
-8
0
-0.5
Time (50μs/div)
Enable Voltage
(top) (1V/div)
3.5
2
2
1
1.9
0
1.8
-1
1.7
-2
1.6
Output Voltage
(bottom) (100mV/div)
6
Inductor Current
(bottom) (500mV/div)
Enable Voltage (top) (2V/div)
Output Voltage (middle) (2V/div)
T = 120°C
T = 100°C
T = 85°C
T = 25°C
100
Input Voltage (V)
-10
Inductor Current
(bottom) (500mA/div)
1.84
Inductor Current
(bottom) (100mA/div)
Output Voltage
(top) (20mV/div)
Typical Characteristics
Time (200μs/div)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
7
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Typical Characteristics
Step-Down Converter Load Transient Response
Step-Down Converter Line Regulation
(VOUT = 1.8V; VIN = 5V; IOUT = 1mA to 600mA; CFF = 0pF)
(VOUT = 1.8V; L = 2.2μH)
1.9
3
1.8
2.5
1.7
2
1.6
1.5
1.5
1
1.4
0.5
1.3
0
1.2
-0.5
1
IOUT = 600mA
IOUT = 300mA
IOUT = 100mA
IOUT = 10mA
IOUT = 0.1mA
0.8
0.6
Accuracy (%)
Output Voltage
(top) (100mV/div)
3.5
Output Current (middle)
Inductor Current (bottom)
(500mA/div)
2
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
2.3
2.7
3.1
3.5
3.9
4.3
4.7
Input Voltage (V)
Time (100μs/div)
LDO Power Supply Rejection Ratio, PSRR
(IOUT2 = 1.8V; BW = 100Hz to 300Hz)
100
Magnitude (dB)
90
80
70
60
50
40
30
20
10
0
102
103
104
105
Frequency (Hz)
8
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
5.1
5.5
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Functional Block Diagram
IN_BUCK
VP
EN
REF
DC-DC
Converter
LX
CE
Control
GND
MODE
OUT_BUCK
VP
REF
EN
IN_LDO
LDO
OUT_LDO
GND
Functional Description
The AAT2749 is a two channel power converter that
operates from a 2.3V to 5.5V power supply. The AAT2749
step-down converters can deliver up to 600mA while the
LDO regulator can deliver up to 300mA. The typical no
load quiescent current is 100μA for both the step-down
converter and LDO regulator.
Step-Down Regulator
The AAT2749 contains a high performance 600mA,
3.0MHz monolithic step-down converter. Typically, a
2.2μH inductor and a 4.7μF ceramic capacitor are recommended (see table of values).
At dropout, the converter duty cycle increases to 100%
and the output voltage tracks the input voltage minus
the RDS(ON) drop of the P-channel high-side MOSFET. The
input voltage range is 2.3V to 5.5V. The converter efficiency has been optimized for all load conditions, ranging
from no load to 600mA.
The internal error amplifier and compensation provides
excellent transient response, load, and line regulation.
Soft start eliminates any output voltage overshoot when
the enable or the input voltage is applied.
Mode Pin
To ensure high efficiency across the load range, the
AAT2749 will automatically shift out of PWM mode in
light load conditions. This mode of operation maintains
high efficiency under light load conditions (typically
<150mA). The MODE pin allows optional fixed frequency
PWM mode for improved noise performance. This maintains constant frequency and low output ripple across all
load conditions.
Control Loop
The AAT2749 step down converter is a peak current
mode step-down converter. The current through the
P-channel MOSFET (high side) is sensed for current loop
control, as well as short circuit and overload protection.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
9
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
trans-conductance voltage error amplifier output.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When disabled, the
AAT2749 is in a low-power, non-switching state. The
total input current during shutdown is less than 1μA.
Current Limit
For overload conditions, the peak input current is limited. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
Applications Information
Step-Down Converter Inductor Selection
The step-down converter uses peak current mode control with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation for the adjustable and
low voltage fixed versions of channel 1 is 0.75A/μs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 2.2μH
inductor.
m=
L=
0.75 · VO 0.75 · 1.8V
A
=
= 0.75
L
1.8μH
μs
0.75 · VO 0.75 · 1.8V
=
= 1.8μH
m
A
0.75 μs
The 300mA LDO regulator is stable with a 2.2μF ceramic output capacitor. The LDO regulator has a current limit
to protect against short circuit conditions.
The inductor should be set equal to the output voltage
numeric value in micro henries (μH). This guarantees
that there is sufficient internal slope compensation.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the saturation characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
Over-Temperature Protection
Step-Down Converter Input Capacitor
Thermal protection completely disables the converters
when internal dissipation becomes excessive. The junction over-temperature threshold is 140°C with 15°C of
hysteresis. Once an over-temperature or over-current
fault conditions is removed, the output voltage automatically recovers.
Select a 2.2μF to 4.7μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and
solve for CIN. The calculated value varies with input voltage and is a maximum when VIN is double the output
voltage.
LDO Regulator
The AAT2749 includes a low-dropout (LDO) linear regulator. The LDO regulator operates from 1.62V to 5.5V;
however, VIN_LDO cannot exceed the input voltage to the
step-down regulator (VIN_BUCK). The linear regulator output voltage is set by internal resistive voltage dividers.
The LDO regulator consumes about 50μA of quiescent
current.
Chip Enable Pin (CE)
Logic High at CE pin enables step-down converter and
LDO.
CIN =
V
VO
· 1- O
VIN
VIN
VPP
- ESR · FS
IO
VO
V
· 1- O
VIN
VIN
10
=
1
for VIN = 2 · VO
4
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
1
CIN(MIN) =
VPP
- ESR · 4 · FS
IO
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper 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:
VO
V
· 1- O
VIN
VIN
IRMS = IO ·
=
D · (1 - D) =
0.52 =
1
2
For VIN = 2 · VO
IRMS(MAX) =
VO
· 1-
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 capacitor. This dampens
the high Q network and stabilizes the system.
Step-Down Converter Output Capacitor
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
V
· 1- O
VIN
VIN
the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain
measurements 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.
IO
2
VO
VIN
The term 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 AAT2749. 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 the Layout section of
this datasheet.
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 inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
The output capacitor limits the output ripple and provides holdup during large load transitions. A 2.2μF to
4.7μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL characteristics necessary for low output ripple.
The output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, 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.
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
2· 3
·
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.
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DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
The maximum output capacitor RMS ripple current is
given by:
IRMS(MAX) =
1
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
2· 3
·
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.
LDO Input Capacitor
Typically, a 1μF or larger capacitor is recommended for
CIN in most applications. A CIN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2749 is physically located more than three centimeters from an input power source, a CIN capacitor will be
needed for stable operation.
CIN should be located as close to the device VIN pin as
possible. CIN values greater than 1μF will offer superior
input line transient response and will assist in maximizing the highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for CIN. There is no specific capacitor
ESR requirement for CIN. However, for 300mA LDO regulator output operation, ceramic capacitors are recommended for CIN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources, such as batteries in portable devices.
LDO Output Capacitor
For proper load voltage regulation and operational stability, a capacitor is required between the VOUT and GND
pins. The COUT capacitor connection to the LDO regulator
ground pin should be as close as possible for maximum
device performance. The AAT2749 LDO has been specifically designed to function with very low ESR ceramic
capacitors. For best performance, ceramic capacitors are
recommended.
Typical output capacitor values for maximum output current conditions range from 1μF to 10μF. Applications
utilizing the exceptionally low output noise and optimum
power supply ripple rejection characteristics of the channel 2 should use 2.2μF or greater for COUT. If desired, COUT
may be increased without limit. In low output current
applications where output load is less than 10mA, the
minimum value for COUT can be as low as 0.47μF.
12
LDO Enable Function
The AAT2749 features an LDO regulator enable/disable
function. This pin (EN) is active high and is compatible
with CMOS logic. To assure the LDO regulator will switch
on, the EN turn-on control level must be greater than
1.5V. The LDO regulator will go into the disable shutdown mode when the voltage on the EN pin falls below
0.6V. If the enable function is not needed in a specific
application, it may be tied to VIN to keep the LDO regulator in a continuously on state. When the LDO regulator
is in shutdown mode, an internal 1.5kΩ resistor is connected between VOUT and GND. This is intended to discharge COUT when the LDO regulator is disabled. The
internal 1.5kΩ has no adverse effect on device turn-on
time.
LDO Short-Circuit Protection
The AAT2749 LDO contains an internal short-circuit protection circuit that will trigger when the output load current exceeds the internal threshold limit. Under shortcircuit conditions, the output of the LDO regulator will be
current limited until the short-circuit condition is removed
from the output or LDO regulator package power dissipation exceeds the device thermal limit.
LDO Thermal Protection
The AAT2749 LDO has an internal thermal protection
circuit which will turn on when the device die temperature exceeds 150°C. The internal thermal protection
circuit will actively turn off the LDO regulator output pass
device to prevent the possibility of over temperature
damage. The LDO regulator output will remain in a shutdown state until the internal die temperature falls back
below the 150°C trip point. The combination and interaction between the short circuit and thermal protection
systems allows the LDO regulator to withstand indefinite
short-circuit conditions without sustaining permanent
damage.
Thermal Calculations
There are three types of losses associated with the
AAT2749 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the synchro-
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DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
nous step-down converter and LDO losses is given by:
PTOTAL =
IOUT12 · (RDS()ON)H · VOUT1 + RDS(ON)L · [VIN1 -VOUT1])
VIN1
+ (tSW · FS · IOUT1 + IQ1) · VIN1 + (VIN2 - VOUT2 ) · IOUT2
IQ1 is the step-down converter and LDO quiescent current respectively. The term tSW is used to estimate the
full load step-down converter switching losses.
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be investigated over the complete input voltage range.
Given the total losses, the maximum junction temperature can be derived from the θJA for the CSP-9 package,
which is 284°C/W.
Layout Considerations
The suggested PCB layout for the AAT2749 is shown in
Figures 2 and 3. The following guidelines should be used
to help ensure a proper layout.
1.
2.
3.
4.
TJ(MAX) = PTOTAL · θJA + TA
5.
The bypass capacitors (C1, C2 and C4) should connect as closely as possible to input and output pin
(Pins 1, 2, and 7) and PGND (Pin 9).
C3 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as
short as possible.
The feedback trace or OUT-BUCK pin (Pin 3) should
be separated from any power trace and connected as
closely as possible to the load point. Sensing along a
high current load trace will degrade DC load regulation.
The resistance of the trace from the load return to
PGND (Pin 9) should be kept to a minimum. This will
help to minimize any error in DC regulation due to
differences in the potential of the internal signal
ground and the power ground.
The pad on the PCB for the CSP-9 package should
use NSMD (non-solder mask defined) configuration
due to its tighter control on the copper etch process.
A pad thickness of less than 1 oz. is recommended
to achieve higher stand-off. A high-density, small
footprint layout can be achieved using an inexpensive, miniature, non-shielded, high DCR inductor.
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13
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
JP2
1
IN_LDO
OUT_LDO
OUT-LDO
2
C2
1μF
VIN-BUCK
7
IN_BUCK
2.3V-5.5V
LX
8
2.2μH
AAT2749
EN
6
CE
4
MODE
L1
1.2V/300mA
C4
2.2μF
OUT_BUCK
3
PGND
9
OUT-BUCK
1.8V/600mA
C3
4.7μF
C1
4.7μF
MODE
U1
C1, C3
C2
C3
L1
U1
5
AAT2749 Skyworks, 300mA LDO and 600mA Buck Converter, CSP-9
Cap, MLC, 4.7μF/10V, 0805
Cap, MLC, 1μF/10V, 0805
Cap, MLC, 2.2μF/6.3V, 0805
LQM2MPN2R2NGO, Murata, 2.2μH, ISAT = 1.2A, DCR = 0.11Ω
Figure 2: AAT2749-IUR Evaluation Board Schematic and Bill of Materials (BOM).
Figure 3: AAT2749-IUR Evaluation
Board Top Layer.
14
Figure 4: AAT2749-IUR Evaluation
Board Bottom Layer.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
AAT2749 Design Example
Specifications
VO1 = 1.8V @ 600mA, Pulsed Load ILOAD = 600mA
VO2 = 1.2V @ 300mA
VIN1 = 5V
FS = 3MHz
TAMB = 85°C in CSP-9 Package
Step-Down Converter Output Inductor
For Murata chip inductor LQM2MPN2R2NG0, 2.2μH, DCR = 0.11Ω.
ΔI =
VOUT1
V
1.8V
1.8V
· 1 - OUT1 =
· 1= 174mA
L · FS
VIN1
2.2μH · 3MHz
5V
IPK1 = IOUT1 +
ΔI
= 600mA + 87mA = 687mA
2
PL1 = IOUT12 · DCR = 600mA2 · 111mΩ = 40mW
Step-Down Converter Output Capacitor
VDROOP = 0.18V (10% Output Voltage)
COUT =
3 · ΔILOAD
3 · 600mA
=
= 3μF; use 4.7μF
0.18V · 3MHz
VDROOP · FS
IRMS(MAX) =
VOUT1 · (VIN(MAX) - VOUT1)
1
1.8V · (5.5V - 2.3V)
·
= 46mArms
=
L · FS · VIN1(MAX)
2 · 3 2.2μH · 3MHz · 5.5V
2· 3
1
·
PRMS = ESR · IRMS2 = 5mΩ · (46mA)2 = 11μW
Step-Down Converter Input Capacitor
Input Ripple VPP = 10mV
CIN1 =
IRMS =
1
VPP
- ESR · 4 · FS
IOUT1
=
1
10mV
- 5mΩ · 4 · 3MHz
600mA
= 7μF; use 4.7μF
IOUT1
= 300mA
2
P = ESR · (IRMS2) = 5mΩ · (300mA)2 = 0.31mW
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15
DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
AAT2749 Losses
All values assume 25°C ambient temperature and thermal resistance of 284°C/W in the CSP-9 package.
PTOTAL =
IOUT12 · (RDS(ON)H · VOUT1 + RDS(ON)L · [VIN1 - VOUT1])
VIN1
+ (tSW · FS · IOUT1 + IQ1) · VIN + (VIN2 - VOUT2) · IOUT2
=
600mA2 · (360mΩ · 1.8V + 200mΩ · [5V - 1.8V])
5V
+ (5ns · 3MHz · 600mA + 100μA) · 5V + (1.8V - 1.2V) · 300mA
PTOTAL = 318mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 25°C + (284°C/W) · 318mW = 115°C
16
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DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-VIN LDO
Ordering Information
Output Voltages
VOUT_BUCK
VOUT_LDO
Package
Part Marking
Part Number (Tape and Reel)
1.8V
1.8V
1.2V
1.0V
WLCSP-9
WLCSP-9
6QXY
7UXY
AAT2749IUR-IE-T1
AAT2749IUR-ID-T1
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information
WLCSP-9
0.300
0.100
0.100
Line 2
Top View
0.500 ± 0.050
0.205 ± 0.025
0.300
1.360 ± 0.035
0.2 (Ref.)
Pin 1 Indication
0.400 BSC
0.800
1.350 ± 0.035
0.170 ± 0.025
0.330 ± 0.025
Line 1
Bottom View
Side View
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17