Datasheet

A S1 3 2 4
1 .5 M H z, 6 0 0m A S yn ch r o n o u s D C/ DC C o n v e r te r
1 General Description
2 Key Features
The AS1324 is a high-efficiency, constant-frequency synchronous
buck converter available in adjustable- and fixed-voltage versions.
The wide input voltage range (2.7V to 5.5V), automatic powersave
mode and minimal external component requirements make the
AS1324 perfect for any single Li-Ion battery-powered application.
High Efficiency: Up to 96%
Typical supply current with no load is 30µA and decreases to ≤1µA
in shutdown mode.
Constant Frequency Operation: 1.5MHz
The AS1324 is available as the standard versions listed in Table 1.
Variable- and Fixed-Output Voltages
Table 1. Standard Versions
No Schottky Diode Required
Output Current: 600mA
Input Voltage Range: 2.7V to 5.5V
Model
Output Voltage
AS1324-AD
Adjustable via External Resistors
AS1324-12
Fixed: 1.2V
AS1324-15
Fixed: 1.5V
Internal Reference: 0.6V
AS1324-18
Fixed: 1.8V
Shutdown Mode Supply Current: ≤1µA
Automatic Powersave Operation
Low Quiescent Current: 30µA
Thermal Protection
An internal synchronous switch increases efficiency and eliminates
the need for an external Schottky diode. The internally fixed
switching frequency (1.5MHz) allows for the use of small surface
mount external components.
5-pin TSOT-23 Package
Very low output voltages can be delivered with the internal 0.6V
feedback reference voltage.
3 Applications
The AS1324 is available in a 5-pin TSOT-23 package.
The device is ideal for mobile communication devices, laptops and
PDAs, ultra-low-power systems, threshold detectors/discriminators,
telemetry and remote systems, medical instruments, or any other
space-limited application with low power-consumption requirements.
Figure 1. Typical Application Diagram – High Efficiency Step
Down Converter
VIN = 2.7V to 5.5V
CIN
4
3
SW
VIN
AS1324-18
10µF
4.7µH
VOUT = 1.8V, 600mA
EN 1
5 VOUT
COUT
AS1324-18
10µF
GND 2
5
1
VOUT
EN
2
www.ams.com/DC-DC_Step-Up/AS1324
SW 3
4 VIN
GND
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AS1324
Datasheet - P i n A s s i g n m e n t s
4 Pin Assignments
Figure 2. Pin Assignments (Top View)
EN
1
GND
2
SW
3
5
AS1324
4
EN
VFB
1
GND
2
SW
3
VIN
5
VOUT
4
VIN
AS1324-12/
AS1324-15/
AS1324-18
4.1 Pin Descriptions
Table 2. Pin Descriptions
Pin Number
Pin Name
Description
Enable Input. Driving this pin above 1.5V enables the device. Driving this pin below 0.3V puts the
device in shutdown mode. In shutdown mode all functions are disabled while SW goes high
impedance, drawing <1µA supply current.
Note: This pin should not be left floating.
1
EN
2
GND
Ground.
3
SW
Switch Node Connection to Inductor. This pin connects to the drains of the internal main and
synchronous power MOSFET switches.
4
VIN
Input Supply Voltage. This pin must be closely decoupled to GND with a ≥ 4.7µF ceramic capacitor.
Connect to any supply voltage between 2.7 to 5.5V.
VFB
Feedback Pin. This pin receives the feedback voltage from the external resistor divider across the
output. (Adjustable voltage variant only.)
5
VOUT
Output Voltage Feedback Pin. An internal resistor divider steps the output voltage down for
comparison to the internal reference voltage. (Fixed voltage variants only.)
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AS1324
Datasheet - A b s o l u t e M a x i m u m R a t i n g s
5 Absolute Maximum Ratings
Stresses beyond those listed in Table 3 may cause permanent damage to the device. These are stress ratings only, and functional operation of
the device at these or any other conditions beyond those indicated in Section 6 Electrical Characteristics on page 4 is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Table 3. Absolute Maximum Ratings
Parameter
Min
Max
Units
VIN to GND
-0.3
6
V
SW, EN, FB to GND
-0.3
VIN
+ 0.3
V
207.4
ºC/W
on PCB
kV
HBM MIL-Std. 883E 3015.7 methods
JEDEC 78
Thermal Resistance ΘJA
ESD
2
Latch-Up
-100
+100
mA
Operating Temperature Range
-40
+85
ºC
Storage Temperature Range
-65
+125
ºC
Package Body Temperature
+260
Junction Temperature
125
ºC
ºC
Comments
The reflow peak soldering temperature (body
temperature) specified is in accordance with IPC/
JEDEC J-STD-020 “Moisture/Reflow Sensitivity
Classification for Non-Hermetic Solid State Surface
Mount Devices”.
The lead finish for Pb-free leaded packages is matte tin
(100% Sn).
Junction temperature (TJ) is calculated from the
ambient temperature (TAMB) and power dissipation
(PD) as:
TJ = TAMB + (PD)(207.4ºC/W)
Moisture Sensitive Level
www.ams.com/DC-DC_Step-Up/AS1324
1
(EQ 1)
Represents an unlimited floor life time
Revision 1.06
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AS1324
Datasheet - E l e c t r i c a l C h a r a c t e r i s t i c s
6 Electrical Characteristics
VIN = EN = 3.6V, VOUT < VIN - 0.5V, TAMB = -40 to +85°C, typ. values @ TAMB = +25ºC (unless otherwise specified).
Table 4. Electrical Characteristics
Symbol
Parameter
Conditions
Min
Typ
VIN
Input Voltage Range
IQ
Quiescent Current
Powersave Mode; VFB = 0.62V or VOUT = 103%,
IOUT = 0mA, TAMB = +25ºC
30
35
ISHDN
Shutdown Current
Shutdown Mode; VEN = 0V, TAMB = +25ºC
0.1
1
0.6
0.615
V
0.1
1
%/V
30
nA
2.7
Max
Units
5.5
V
µA
Regulation
1
AS1324, IOUT = 100mA
VFB
Regulated Feedback Voltage
∆VFB
Reference Voltage
Line Regulation
VIN = 2.7V to 5.5V
IVFB
Feedback Current
TAMB = +25ºC
0.585
-30
2
VOUT
Regulated Output Voltage
AS1324-AD, IOUT = 100mA
VFB
AS1324-12, IOUT = 100mA
1.164
1.20
1.236
AS1324-15, IOUT = 100mA
1.455
1.50
1.545
AS1324-18, IOUT = 100mA
1.746
1.80
1.854
1
V
∆VOUT
Output Voltage
Line Regulation
VIN = 2.7 to 5.5V
0.1
VLOADREG
Output Voltage
Load Regulation
IOUT = 0 to 100mA
0.02
IPK
Peak Inductor Current
VIN = 3V, VFB = 0.5V or VOUT = 90%, TAMB =
25ºC
RPFET
P-Channel FET RDS(ON)
ILSW = 100mA
0.4
Ω
RNFET
N-Channel FET RDS(ON)
ILSW = -100mA
0.35
Ω
ILSW
SW Leakage
VEN = 0V, VSW = 0V or 5V
±0.01
±1
µA
1
1.5
V
±0.01
±1
µA
1.5
1.8
MHz
%/V
%/mA
DC-DC Switches
0.5
0.75
1
A
Control Inputs
VEN
EN Threshold
IEN
EN Leakage Current
0.3
Oscillator
fOSC
Oscillator Frequency
VFB = 0.6V or VOUT = 100%
1.2
VFB = 0V or VOUT = 0V, TAMB = 25ºC
115
kHz
1. The device is tested in a proprietary test mode where VFB is connected to the output of the error amplifier.
2. Please see Feedback Resistor Selection on page 13 for resistor values.
Note: All limits are guaranteed. The parameters with min and max values are guaranteed with production tests or SQC (Statistical Quality
Control) methods.
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AS1324
Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s
7 Typical Operating Characteristics
Parts used for measurement: 4.7µH (MOS6020-472) Inductor, 10µF (GRM188R60J106ME47) CIN and COUT.
Figure 4. Efficiency vs. Output Current; VOUT = 1.2V
95
100
90
95
85
90
Efficiency (%) .
Efficiency (%) .
Figure 3. Efficiency vs. Input Voltage; VOUT = 1.8V
80
75
70
65
IOUT = 600mA
60
80
75
70
65
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
60
IOUT = 100mA
55
85
IOUT = 10mA
55
IOUT = 1mA
50
50
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
1
Input Voltage (V)
100
95
95
90
90
85
80
75
70
65
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
55
1000
85
80
75
70
65
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
60
55
50
50
1
10
100
1000
1
Output Current (mA)
100
1000
Figure 8. Efficiency vs. Output Current; VOUT = 3.3V
100
95
95
90
90
85
85
Efficiency (%) .
100
80
75
70
65
VIN = 3.7V
60
10
Output Current (mA)
Figure 7. Efficiency vs. Output Current; VOUT = 2.5V
Efficiency (%) .
100
Figure 6. Efficiency vs. Output Current; VOUT = 1.8V
100
Efficiency (%) .
Efficiency (%) .
Figure 5. Efficiency vs. Output Current; VOUT = 1.5V
60
10
Output Current (mA)
80
75
70
65
60
VIN = 4.2V
55
VIN = 5.5V
50
55
VIN = 4.2V
VIN = 5.5V
50
1
10
100
1000
1
Output Current (mA)
www.ams.com/DC-DC_Step-Up/AS1324
10
100
1000
Output Current (mA)
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AS1324
Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s
Figure 9. Switching Frequency vs. Supply Voltage
Figure 10. Switching Frequency vs. Temperature
1.6
Switching Frequency (MHz)
Switching Frequency (MHz) .
.
1.6
1.55
1.5
1.45
1.4
2.7
3.1
3.5
3.9
4.3
4.7
5.1
1.55
1.5
1.45
1.4
-45 -30 -15
5.5
Input Voltage (V)
0
15
30
45
60
75
90
Temperature (°C)
Figure 11. Feedback Voltage vs. Temperature
Figure 12. Output Voltage vs. Input Voltage
0.61
2
Output Voltage (V) .
Feedback Voltage (V) .
1.95
0.605
0.6
0.595
1.9
1.85
1.8
1.75
1.7
IOUT = 600mA
IOUT = 100mA
IOUT = 10mA
IOUT = 1mA
IOUT = 100µA
1.65
0.59
-45 -30 -15
1.6
0
15
30
45
60
75
2.7
90
3.1
Temperature (C°)
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Figure 13. VOUT vs. IOUT; VOUTNOM = 1.2V
Figure 14. VOUT vs. IOUT; VOUTNOM = 1.5V
1.6
1.3
Vin=2.5V
Vin=2.5V
Vin=2.7V
Vin=2.7V
Vin=5.5V
1.25
Output Voltage (V) .
Output Voltage (V) .
3.5
Vin=5.5V
1.55
1.2
1.5
1.45
1.15
1.4
1.1
0
100
200
300
400
500
0
600
www.ams.com/DC-DC_Step-Up/AS1324
100
200
300
400
500
600
Output Current (mA)
Output Current (mA)
Revision 1.06
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AS1324
Datasheet - Ty p i c a l O p e r a t i n g C h a r a c t e r i s t i c s
Figure 16. Quiescent Current vs. Temperature
50
50
45
45
Quiescent Current (µA) .
40
35
30
25
20
15
10
5
40
35
30
25
20
15
10
5
0
4.7
5.1
0
-45 -30 -15
5.5
Input Voltage (V)
15
30
45
60
75
90
VOUT
ISW
IOUT
200mV/DIV
500mA/DIV
Figure 18. Load Step 10mA to 200mA
600mA/DIV
ISW
VOUT
Figure 17. Load Step 0mA to 600mA
IOUT
0
Temperature (°C)
200mV/DIV
4.3
500mA/DIV
3.9
200mA/DIV
3.5
500µs/DIV
500µs/DIV
Figure 20. Powersave Mode
VOUT
ISW
VSW
1V/DIV
560mA/DIV
ISW
VOUT
EN
5V/DIV
Figure 19. Startup
1ms/DIV
www.ams.com/DC-DC_Step-Up/AS1324
5V/DIV
3.1
100mV/DIV
2.7
200mA/DIV
Quiescent Current (µA) .
Figure 15. Quiescent Current vs. Input Voltage
5µs/DIV
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AS1324
Datasheet - D e t a i l e d D e s c r i p t i o n
8 Detailed Description
The AS1324 is a high-efficiency buck converter that uses a constant-frequency current-mode architecture. The device contains two internal
MOSFET switches and is available in adjustable- and fixed-output voltage versions.
Figure 21. AS1324 - Block Diagram
Ramp
Compensator
–
VIN
+
OSCN
VIN
4
ICOMP
OSC
CIN
10µF
Frequency
Shift
5
VOUT/VFB
FB
R2
AS1324
+
Error
Amp
–
0.6V
R1
–
OVDET
0.6V +
∆VOVL
+
PMOS
Digital
Logic
AntiShoot
Through
3
–
1
EN
0.6V
Reference
0.6V ∆VOVL
SW
4.7µH
VOUT
COUT
10µF
+
NMOS
+
IRCMP
Shutdown
–
2
GND
Not applicable to AS1324
AS1324-12: R1 + R2 = 600kΩ
AS1324-15: R1 + R2 = 750kΩ
AS1324-18: R1 + R2 = 900kΩ
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AS1324
Datasheet - D e t a i l e d D e s c r i p t i o n
8.1 Main Control Loop
During PWM operation the converters use a 1.5MHz fixed-frequency, current-mode control scheme. Basis of the current-mode PWM controller is
an open-loop, multiple input comparator that compares the error-amp voltage feedback signal against the sum of the amplified current-sense
signal and the slope-compensation ramp. At the beginning of each clock cycle, the internal high-side PMOS turns on until the PWM comparator
trips. During this time the current in the inductor ramps up, sourcing current to the output and storing energy in the inductor’s magnetic field.
When the PMOS turns off, the internal low-side NMOS turns on. Now the inductor releases the stored energy while the current ramps down, still
providing current to the output. The output capacitor stores charge when the inductor current exceeds the load and discharges when the inductor
current is lower than the load. Under overload conditions, when the inductor current exceeds the current limit, the high-side PMOS is turned off
and the low-side NMOS remains on until the next clock cycle.
When the PMOS is off, the NMOS is turned on until the inductor current starts to reverse (as indicated by the current reversal comparator
(IRCMP)), or the next clock cycle begins. The IRCMP detects the zero crossing.
The peak inductor current (IPK) is controlled by the error amplifier. When IOUT increases, VFB decreases slightly relative to the internal 0.6V
reference, causing the error amplifier’s output voltage to increase until the average inductor current matches the new load current.
The over-voltage detection comparator (OVDET) guards against transient overshoots by turning the main switch off and keeping it off until the
transient is removed.
8.2 Powersave Operation
The AS1324 uses an automatic powersave mode where the peak inductor current (IPK) is set to approximately 200mA while independent of the
output load. In powersave mode, load current is supplied solely from the output capacitor. As the output voltage drops, the error amplifier output
rises above the powersave threshold signaling to switch into PWM fixed frequency mode and turn the PMOS on. This process repeats at a rate
determined by the load demand.
Each burst event can last from a few cycles at light loads to almost continuous cycling (with short sleep intervals) at moderate loads. In between
bursts, the power MOSFETs are turned off, as is any unneeded circuitry, reducing quiescent current to 30µA.
8.3 Short-Circuit Protection
In cases where the AS1324 output is shorted to ground, the oscillator frequency (fOSC) is reduced to 1/13 the nominal frequency (≅ 115kHz).
This frequency reduction ensures that the inductor current has more time to decay, thus preventing runaway conditions. fOSC will progressively
increase to 1.5MHz when VFB/VOUT > 0V.
8.4 Shutdown
Connecting EN to GND or logic low places the AS1324 in shutdown mode and reduces the supply current to 0.1µA. In shutdown the control
circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance disconnecting the input from the output. The output
capacitance and load current determine the voltage decay rate. For normal operation connect EN to VIN or logic high.
Note: Pin EN should not be left floating.
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9 Application Information
The AS1324 is perfect for mobile communications equipment like cell phones and smart phones, digital cameras and camcorders, portable MP3
and DVD players, PDA’s and palmtop computers and any other handheld instruments.
Figure 22. Single Li-Ion 1.2V/600mA Regulator for High-Efficiency
VIN
2.7 to 4.2V
4
CIN
2.2µF
4.7µH
3
VIN
SW
COUT
10µF
AS1324
22pF
301kΩ
5
1
R2
VFB
EN
2
VOUT
1.2V
600mA
301kΩ
R1
GND
Figure 23. 5V Input to 3.3V/600mA Buck Regulator
VIN
5V
4
CIN
4.7µF
4.7µH
3
VIN
COUT
10µF
SW
AS1324
22pF
301kΩ
5
1
VFB
EN
VOUT
3.3V
600mA
R2
R1
2 GND
66.5kΩ
Figure 24. Single Li-Ion 1.5V/600mA Regulator for High-Efficiency
VIN
2.7 to 4.2V
4
CIN
4.7µF
3
VIN
SW
AS1324-15
4.7µH
COUT
10µF
VOUT
1.5V
600mA
5
1
VOUT
EN
2 GND
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
Figure 25. Single Li-Ion 1.8V/600mA Regulator for Low Output Ripple
VIN
2.7 to 4.2V
4
CIN
10µF
3
VIN
4.7µH
SW
AS1324-18
COUT
22µF
VOUT
1.8V
600mA
5
1
VOUT
EN
2
GND
9.1 External Component Selection
9.2 Inductor Selection
For most applications the value of the external inductor should be in the range of 2.2 to 6.8µH as the inductor value has a direct effect on the
ripple current. The selected inductor must be rated for its DC resistance and saturation current. The inductor ripple current (∆IL) decreases with
higher inductance and increases with higher VIN or VOUT.
In Equation (EQ 2) the maximum inductor current in PWM mode under static load conditions is calculated. The saturation current of the inductor
should be rated higher than the maximum inductor current as calculated with Equation (EQ 3). This is recommended because the inductor
current will rise above the calculated value during heavy load transients.
V OUT
1 – -------------V IN
∆I L = V OUT × -----------------------L×f
(EQ 2)
∆I L
I LMAX = I OUTMAX + -------2
(EQ 3)
Where:
f = Switching Frequency (1.5 MHz typical)
L = Inductor Value
ILmax = Maximum Inductor current
∆IL = Peak to Peak inductor ripple current
The recommended starting point for setting ripple current is ∆IL = 240mA (40% of 600mA).
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Thus, a 720mA rated inductor should be sufficient for most applications (600mA + 120mA). A easy and fast approach is to select the inductor
current rating fitting to the maximum switch current limit of the converter.
Note: For highest efficiency, a low DC-resistance inductor is recommended.
Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage ripple, greater core losses,
and lower output current capability.
The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the losses in the dc resistance and
the following frequency-dependent components:
1.
2.
3.
4.
The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
Additional losses in the conductor from the skin effect (current displacement at high frequencies)
Magnetic field losses of the neighboring windings (proximity effect)
Radiation losses
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
Table 5. Recommended Inductors
Part Number
L
DCR
Current Rating
Dimensions (L/W/T)
LQH32CN2R2M33
2.2µH
97mΩ
790mA
3.2x2.5x2.0mm
LQH32CN4R7M33
4.7µH
150mΩ
650mA
3.2x2.5x2.0mm
LPS3008-222MLC
2.2µH
175mΩ
1100mA
3.1x3.1x0.8mm
LPS3015-222MLC
2.2µH
110mΩ
2000mA
3.1x3.1x1.5mm
MOS6020-222MLC
2.2µH
35mΩ
3260mA
6.0x6.8x2.4mm
MOS6020-472MLC
4.7µH
50mΩ
1820mA
6.0x6.8x2.4mm
CDRH3D16NP-2R2N
2.2µH
72mΩ
1200mA
4.0x4.0x1.8mm
CDRH3D16ND-4R7N
4.7µH
105mΩ
900mA
4.0x4.0x1.8mm
Manufacturer
Murata
www.murata.com
Coilcraft
www.coilcraft.com
Sumida
www.sumida.com
Figure 26. Efficiency Comparison of Different Inductors, VIN = 2.7V, VOUT = 1.8V and 1.2V
95
95
VOUT = 1.8V
90
Efficiency (%) .
90
Efficiency (%) .
VOUT = 1.2V
85
80
85
80
75
75
LQH32CN2R2
LPS3015-222
LQH32CN4R7
LQH32CN2R2
LPS3015-222
LQH32CN4R7
LPS3008-222
M OS6020-222
M OS6020-472
70
70
1
10
100
LPS3008-222
M OS6020-222
M OS6020-472
1000
1
10
100
1000
Output Current (mA)
Output Current (mA)
9.3 Output Capacitor Selection
The advanced fast-response voltage mode control scheme of the AS1324 allows the use of tiny ceramic capacitors. Because of their lowest
output voltage ripple low ESR ceramic capacitors are recommended. X7R or X5R dielectric output capacitor are recommended.
At high load currents, the device operates in PWM mode and the RMS ripple current is calculated as:
VOUT
1 – -------------V IN
1
IRMSC
= V OUT × ------------------------ × ----------------OUT
L×f
2× 3
(EQ 4)
While operating in PWM mode the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the
voltage ripple caused by charging and discharging the output capacitor:
V OUT
1 – -------------(EQ 5)
V IN
1
∆VOUT = V OUT × ------------------------ ×  -------------------------------- + ESR
 8 × COUT × f

L×f
Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high voltage rating, and low ESR
make them ideal for switching regulator applications. Because the AS1324 control loop is not dependant on the output capacitor ESR for stable
operation, ceramic capacitors can be used to achieve very low output ripple and accommodate small circuit size.
At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output capacitor and inductor
value used. Larger output capacitor and inductor values minimize the voltage ripple in powersave mode and tighten DC output accuracy in
powersave mode.
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9.4 Input Capacitor Selection
In continuous mode, the source current of the PMOS is a square wave of the duty cycle VOUT/VIN. To prevent large voltage transients while
minimizing the interference with other circuits caused by high input voltage spikes, a low ESR input capacitor sized for the maximum RMS
current must be used. The maximum RMS capacitor current is given as:
V OUT × ( VIN – V OUT )
IRMS = IMAX × -----------------------------------------------------------VIN
(EQ 6)
where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current, IMAX = ILIM - ∆IL/2
This formula has a maximum at VIN = 2VOUT where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because
even significant deviations only provide negligible affects.
The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small ceramic input capacitors.
When a small ceramic capacitor is used at the input, and the power is being supplied through long wires, such as from a wall adapter, a load step
at the output, or VIN step on the input, can induce ringing at the VIN pin. This ringing can then couple to the output and be mistaken as loop
instability, or could even damage the part by exceeding the maximum ratings.
9.4.1
Ceramic Input and Output Capacitors
When choosing ceramic capacitors for CIN and COUT, the X5R or X7R dielectric formulations are recommended. These dielectrics have the
best temperature and voltage characteristics for a given value and size. Y5V and Z5U dielectric capacitors, aside from their wide variation in
capacitance over temperature, become resistive at high frequencies and therefore should not be used.
Table 6. Recommended Input and Output Capacitor
Part Number
C
TC Code
Rated Voltage
Dimensions (L/W/T)
JMK212BJ226MG-T
22µF
X5R
6.3V
0805
GRM188R60J106ME47
10µF
X5R
6.3V
0603
GRM21BR71A475KA73
4.7µF
X7R
10V
0805
Manufacturer
Taiyo Yuden
www.t-yuden.com
Murata
www.murata.com
Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended that either a higher input
capacity or a capacitance with a higher rated voltage is used.
9.5 Feedback Resistor Selection
In the AS1324-AD, the output voltage is set by an external resistor divider connected to VFB (see Figure 27). This circuitry allows for remote
voltage sensing and adjustment.
Figure 27. Setting the AS1324 Output Voltage
0.6V ≤ VOUT ≤ 5.5V
R2
5
AS1324
R1<<R2
VFB
R1
2
GND
Resistor values for the circuit shown in Figure 27 can be calculated as:
R2
V OUT = 0,6 × 1 + ------R1
(EQ 7)
The output voltage can be adjusted by selecting different values for R1 and R2. For R1 a value between 10kΩ and 500kΩ is recommended. A
higher resistance of R1 and R2 will result in a lower leakage current at the output. It is recommended to keep VIN 500mV higher than VOUT.
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Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9.6 Efficiency
The efficiency of a switching regulator is equivalent to:
Efficiency = (POUT/PIN)100%
(EQ 8)
For optimum design, an analysis of the AS1324 is needed to determine efficiency limitations and to determine design changes for improved
efficiency. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
(EQ 9)
Where:
L1, L2, L3, etc. are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce losses, those four main sources should be considered for efficiency calculation:
9.6.1
Input Voltage Quiescent Current Losses
The VIN current is the DC supply current given in the electrical characteristics which excludes MOSFET driver and control currents. VIN current
results in a small (<0.1%) loss that increases with VIN, even at no load. The VIN quiescent current loss dominates the efficiency loss at very low
load currents.
9.6.2
I²R Losses
Most of the efficiency loss at medium to high load currents are attributed to I²R loss, and are calculated from the resistances of the internal
switches (RSW) and the external inductor (RL). In continuous mode, the average output current flowing through inductor L is split between the
internal switches. Therefore, the series resistance looking into the SW pin is a function of both NMOS & PMOS RDS(ON) as well as the duty
cycle (DC) and can be calculated as follows:
RSW = (RDS(ON)PMOS)(DC) + (RDS(ON)NMOS)(1 – DC)
(EQ 10)
The RDS(ON) for both MOSFETs can be obtained from the Electrical Characteristics on page 4. Thus, to obtain I²R losses calculate as follows:
I²R losses = IOUT²(RSW + RL)
9.6.3
(EQ 11)
Switching Losses
The switching current is the sum of the control currents and the MOSFET driver. The MOSFET driver current results from switching the gate
capacitance of the power MOSFETs. If a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VIN to
ground. The resulting dQ/dt is a current out of VIN that is typically much larger than the DC bias current. In continuous mode:
IGC = f(QPMOS + QNMOS)
(EQ 12)
Where: QPMOS and QNMOS are the gate charges of the internal MOSFET switches.
The losses of the gate charges are proportional to VIN and thus their effects will be more visible at higher supply voltages.
9.6.4
Other Losses
Basic losses in the design of a system should also be considered. Internal battery resistances and copper trace can account for additional
efficiency degradations in battery operated systems. By making sure that CIN has adequate charge storage and very low ESR at the given
switching frequency, the internal battery and fuse resistance losses can be minimized. CIN and COUT ESR dissipative losses and inductor core
losses generally account for less than 2% total additional loss.
9.7 Thermal Shutdown
Due to its high-efficiency design, the AS1324 will not dissipate much heat in most applications. However, in applications where the AS1324 is
running at high ambient temperature, uses a low supply voltage, and runs with high duty cycles (such as in dropout) the heat dissipated may
exceed the maximum junction temperature of the device.
As soon as the junction temperature reaches approximately 150ºC the AS1324 goes in thermal shutdown. In this mode the internal PMOS &
NMOS switch are turned off. The device will power up again, as soon as the temperature falls below +145°C again.
9.8 Checking Transient Response
The main loop response can be evaluated by examining the load transient response. Switching regulators normally take several cycles to
respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equivalent to:
VDROP = ∆IOUT x ESR
(EQ 13)
Where:
ESR is the effective series resistance of COUT.
∆IOUT also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its
steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem.
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9.9 Design Example
Figure 28 shows the AS1324 used in a single lithium-ion (3.7V typ) battery-powered mobile phone application. The load current requirement is
600mA (max) but most of the time the device will require only 2mA (standby mode current).
Figure 28. Design Example
VIN
3.7V
4
CIN
4.7µF
CER
2.2µH
3
VIN
SW
AS1324
22pF
VFB
2 GND
VOUT
2.2V
1MΩ
5
1
EN
COUT
10µF
CER
R1
R2
375kΩ
For the circuit shown in Figure 28, efficiency at low- and high-load currents is an important consideration when selecting the value for the
external inductor, which is calculated as:
VOUT
V OUT
(EQ 14)
L = -------------- ×  1 – --------------
f∆IL 
V IN 
From (EQ 14), substituting VOUT = 2.2V, VIN = 3.7V, ∆IL = 240mA and f = 1.5MHz gives:
2,2V
2,2V
L = ---------------------------------------------------- ×  1 – ------------ = 2,48µH
( 1,5MHz × 240mA ) 
3 ,7V
(EQ 15)
Therefore, a standard 2.2µH inductor should be used for this design.
For best overall efficiency use an inductor with a rating of 720mA or greater and less than 0.2Ω series resistance. CIN will require an RMS
current rating of at least 0.3A ≅ ILOAD(MAX)/2, whereas COUT will require an ESR of less than 0.25Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, select the value for R1 = 375kΩ. R2 can then be calculated from (EQ 7) to be:
R2 = (VOUT/0.6 - 1)375k = 1000kΩ
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Datasheet - A p p l i c a t i o n I n f o r m a t i o n
9.10 Layout Considerations
The AS1324 requires proper layout and design techniques for optimum performance.
The power traces (GND, SW, and VIN) should be kept as short, direct, and wide as is practical.
Pin VFB (AS1324 only) should be connected directly to the feedback resistors (R1 and R2). A potentiometer as replacement for R1 and R2
should be avoided to minimize the output voltage ripple and to maintain the stability of the regulator.
The resistive divider (R1/R2) must be connected between the positive plate of COUT and ground.
The positive plate of CIN should be connected as close to VIN as is practical since CIN provides the AC current to the internal power MOSFETs.
Switching node SW should be kept far away from the sensitive VFB node.
The negative plates of CIN and COUT should be kept as close to each other as is practical. A starpoint to Ground is recommended.
Figure 29. AS1324 Basic PCB Layout
R1
VIN
Via to VIN
1
VOUT
2
L1
SW
R2
Via to GND
CFWD
Via to VOUT
5
AS1324
3
4
CIN
COUT
GND
Figure 30. AS1324 Basic Diagram
High Current Path
1
EN
2
GND
5
VFB
AS1324
R2
COUT
VOUT
L1
3
SW
4
VIN
R1
CFWD
CIN
VIN
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AS1324
Datasheet - A p p l i c a t i o n I n f o r m a t i o n
Figure 31. AS1324-18 Basic PCB Layout
Via to VIN
VIN
Via to VOUT
1
VOUT
2
L1
SW
5
AS1324-18
3
4
CIN
COUT
GND
Figure 32. AS1324-18 Basic Diagram
High Current Path
1
EN
5
VOUT
AS1324-18
2
GND
COUT
VOUT
L1
3
SW
4
VIN
CIN
VIN
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AS1324
Datasheet - P a c k a g e D r a w i n g s a n d M a r k i n g s
10 Package Drawings and Markings
The device is available in an 5-pin TSOT-23 package.
Figure 33. 5-pin TSOT-23 Package
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Datasheet - P a c k a g e D r a w i n g s a n d M a r k i n g s
Figure 34. 5-pin TSOT-23 Marking
Pin1
Top
ZZZZ
Bottom
XXXX
Pin1
Package Code:
ZZZZ - Marking
XXXX - encoded Datecode
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AS1324
Datasheet
11 Ordering Information
The device is available as the following standard versions.
Table 7. Ordering Information
Ordering Code
Marking
Output
Description
Delivery Form
Package
AS1324-BTTT-AD
ASKR
adjustable
1.5MHz, 600mA Synchronous DC/DC
Converter
Tape and Reel
5-pin TSOT-23
AS1324-BTTT-12
ASKT
1.2V
1.5MHz, 600mA Synchronous DC/DC
Converter
Tape and Reel
5-pin TSOT-23
AS1324-BTTT-15
ASKU
1.5V
1.5MHz, 600mA Synchronous DC/DC
Converter
Tape and Reel
5-pin TSOT-23
AS1324-BTTT-18
ASKS
1.8V
1.5MHz, 600mA Synchronous DC/DC
Converter
Tape and Reel
5-pin TSOT-23
Note: All products are RoHS compliant.
Buy our products or get free samples online at ICdirect: http://www.ams.com/ICdirect
Technical Support is found at http://www.ams.com/Technical-Support
For further information and requests, please contact us mailto:[email protected]
or find your local distributor at http://www.ams.com/distributor
ams
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Datasheet - O r d e r i n g I n f o r m a t i o n
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to designing this product into a system, it is necessary to check with ams AG for current information. This product is intended for use in normal
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