Analogic AAT2158 1.5a low noise step-down converter Datasheet

PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
General Description
Features
The AAT2158 SwitchReg is a 1.5A step-down converter
with an input voltage range of 2.4V to 5.5V and an
adjustable output voltage from 0.6V to VIN. The 1.4MHz
switching frequency enables the use of small external
components. The small footprint and high efficiency make
the AAT2158 an ideal choice for portable applications.
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The AAT2158 delivers 1.5A maximum output current
while consuming only 42µA of no-load quiescent current.
Ultra-low RDS(ON) integrated MOSFETs and 100% duty
cycle operation make the AAT2158 an ideal choice for
high output voltage, high current applications which
require a low dropout threshold.
The AAT2158 provides excellent transient response and
high output accuracy across the operating range. No
external compensation components are required.
1.5A Maximum Output Current
Input Voltage: 2.4V to 5.5V
Output Voltage: 0.6V to VIN
Up to 95% Efficiency
Low Noise Light Load Mode
42µA No Load Quiescent Current
No External Compensation Required
1.4MHz Switching Frequency
100% Duty Cycle Low-Dropout Operation
Internal Soft Start
Over-Temperature and Current Limit Protection
<1µA Shutdown Current
16-Pin 3x3mm QFN Package
Temperature Range: -40°C to +85°C
Applications
The AAT2158 maintains high efficiency throughout the
load range. The AAT2158’s unique architecture produces
reduced ripple and spectral noise. Over-temperature and
short-circuit protection safeguard the AAT2158 and system components from damage.
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The AAT2158 is available in a Pb-free, space-saving
16-pin 3x3mm QFN package. The product is rated over
an operating temperature range of -40°C to +85°C.
Cellular Phones
Digital Cameras
Hard Disk Drives
MP3 Players
PDAs and Handheld Computers
Portable Media Players
USB Devices
Typical Application
U1
AAT2158
3.3V
12
11
10
C1
10µF
7
9
6
8
5
2158.2008.10.1.5
VP
FB
VP
LX
VP
LX
EN
LX
VCC
N/C
N/C
PGND
N/C
PGND
SGND PGND
2.5V
4
R3
187k
15
14
13
L1
2.2µH
16
3
2
R4
59k
C3
22µF
1
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PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Pin Descriptions
Pin #
Symbol
1, 2, 3
PGND
4
FB
5
6, 8, 16
SGND
N/C
7
EN
9
10, 11, 12
VCC
VP
13, 14, 15
LX
EP
Function
Main power ground return pin. Connect to the output and input capacitor return. (See board layout rules.)
Feedback input pin. For an adjustable output, connect an external resistive divider to this pin. For fixed
output voltage versions, FB is the output pin of the converter.
Signal ground. Connect the return of all small signal components to this pin. (See board layout rules.)
Not internally connected.
Enable input pin. A logic high enables the converter; a logic low forces the AAT1158 into shutdown mode
reducing the supply current to less than 1µA. The pin should not be left floating.
Bias supply. Supplies power for the internal circuitry. Connect to input power. Input supply voltage for the converter power stage. Must be closely decoupled to PGND.
Connect inductor to these pins. Switching node internally connected to the drain of both high- and low-side
MOSFETs.
Exposed paddle (bottom); connect to PGND directly beneath package.
Pin Configuration
QFN33-16
(Top View)
LX
LX
LX
N/C
13
14
15
16
PGND
PGND
PGND
FB
1
12
2
11
3
10
4
9
VP
VP
VP
VCC
8
7
6
5
N/C
EN
N/C
SGND
2
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PRODUCT DATASHEET
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SwitchRegTM
AAT2158
1.5A Low Noise Step-Down Converter
Absolute Maximum Ratings1
Symbol
VCC, VP
VLX
VFB
VEN
TJ
Description
VCC, VP to GND
LX to GND
FB to GND
EN to GND
Operating Junction Temperature Range
Value
Units
6
-0.3 to VP + 0.3
-0.3 to VCC + 0.3
-0.3 to -6
-40 to150
V
V
V
V
°C
Value
Units
50
4.2
2.0
°C/W
°C/W
W
Value
Units
-40 to 85
°C
Thermal Characteristics
Symbol
qJA
qJC
PD
Description
Maximum Thermal Resistance
Maximum Thermal Resistance
Maximum Power Dissipation (TA = 25°C)2, 3
Recommended Operating Conditions
Symbol
TA
Description
Ambient Temperature Range
1. Stresses above those listed in Absolute Maximum Ratings may cause 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. Mounted on a demo board (FR4, in still air). Exposed pad must be mounted to PCB.
3. Derate 20mW/°C above 25°C.
2158.2008.10.1.5
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PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Electrical Characteristics1
VIN = 3.6V; TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C.
Symbol
Description
VIN
VOUT
Input Voltage
Output Voltage Range
VUVLO
UVLO Threshold
VOUT
IQ
ISHDN
ILIM
RDS(ON)H
RDS(ON)L
DVLOADREG
DVLINEREG/DVIN
Conditions
Typ
2.4
0.6
IFB
FOSC
TS
TSD
THYS
Output Voltage Tolerance
Quiescent Current
Shutdown Current
Current Limit
High Side Switch On-Resistance
Low Side Switch On-Resistance
Load Regulation
Line Regulation
Feedback Threshold Voltage Accuracy
(Adjustable Version)
FB Leakage Current Internal Oscillator Frequency
Start-Up Time
Over-Temperature Shutdown Threshold
Over-Temperature Shutdown Hysteresis
VIL
VIH
IEN
Enable Threshold Low
Enable Threshold High
Enable Leakage Current
VFB
Min
VIN Rising
Hysteresis
VIN Falling
IOUT = 0A to 1.5A, VIN = 2.4V to 5.5V
No Load
VEN = GND
Max
Units
5.5
VIN
2.4
3.0
90
1.0
V
V
V
mV
V
%
µA
µA
A
W
W
%
%/V
0.609
V
0.2
1.68
µA
MHz
µs
°C
°C
0.6
V
V
µA
250
1.8
-3.0
42
1.8
0.120
0.085
0.5
0.2
ILOAD = 0A to 1.5A
VIN = 2.4V to 5.5V
No Load, TA = 25°C
0.591
0.60
1.12
1.4
150
140
15
VOUT = 1.0V
From Enable to Output Regulation
EN
VIN = VEN = 5.5V
1.4
-1.0
1.0
1. The AAT2158 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correlation with statistical process controls.
4
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PRODUCT DATASHEET
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SwitchRegTM
AAT2158
1.5A Low Noise Step-Down Converter
Typical Characteristics
Efficiency vs. Output Current
Load Regulation
(VOUT = 3.3V)
100
1.00
90
0.75
VOUT Error (%)
Efficiency (%)
(VOUT = 3.3V)
80
VIN = 4.2V
70
VIN = 5.0V
VIN = 3.6V
60
50
VIN = 5.0V
0.50
VIN = 4.2V
0.25
0.00
-0.25
VIN = 3.6V
-0.50
-0.75
40
-1.00
0.1
30
0
1
10
100
1000
10000
1
10
Output Current (mA)
Efficiency vs. Output Current
Load Regulation
(VOUT = 1.8V)
(VOUT = 1.8V)
10000
1.00
VIN = 2.7V
80
70
VIN = 4.2V
0.75
VOUT Error (%)
90
Efficiency (%)
1000
Output Current (mA)
100
VIN = 4.2V
VIN = 3.6V
60
50
40
0.50
VIN = 3.6V
0.25
0.00
-0.25
VIN = 2.7V
-0.50
-0.75
-1.00
30
0
1
10
100
1000
10000
0.1
1
10
Output Current (mA)
100
1000
10000
1000
10000
Output Current (mA)
Efficiency vs. Output Current
Load Regulation
(VOUT = 1.2V)
(VOUT = 1.2V)
1.00
100
90
0.75
VIN = 2.7V
VOUT Error (%)
Efficiency (%)
100
80
70
VIN = 4.2V
VIN = 3.6V
60
50
40
VIN = 4.2V
0.50
0.25
VIN = 2.7V
0.00
-0.25
VIN = 3.6V
-0.50
-0.75
-1.00
30
0
1
10
100
1000
10000
Output Current (mA)
2158.2008.10.1.5
0.1
1
10
100
Output Current (mA)
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PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Typical Characteristics
Quiescent Current vs. Input Voltage
Output Voltage vs. Temperature
(VOUT = 1.8V; No Load)
(VOUT = 1.8V; IOUT = 1A)
1.5
70
Output Voltage Error (%)
Quiescent Current (µA)
80
85°C
60
50
25°C
40
30
-40°C
20
10
0
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
-40 -30 -20 -10
0
Input Voltage (V)
1.79
25°C
1.77
-40°C
1.75
1.74
1.73
2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Switching Frequency (MHz)
Output Voltage (V)
1.80
1.76
90
1.48
1.46
1.44
1.42
-40 -30 -20 -10
0
10
20
30
40
50
60
70
80
90
Temperature (°C)
Load Transient Response
2.4
0.10
2.2
0.00
2.0
-0.10
1.8
-0.20
1.6
-0.30
1.4
-0.40
1.2
-0.50
1.0
-0.60
0.8
0.20
2.4
0.10
2.2
0.00
2.0
-0.10
1.8
-0.20
1.6
-0.30
1.4
-0.40
1.2
-0.50
1.0
-0.60
0.8
Output Current
(bottom) (A)
0.20
Output Voltage (AC coupled)
(top)(mV)
(VOUT = 1.8V; CFF = 100pF)
Output Current
(bottom) (A)
Output Voltage (AC coupled)
(top)(mV)
80
1.5
Time (100µs/div)
6
70
1.52
(VV
1.8V)
OUT
IN = 3.6V
VIN = 3.6V
60
1.54
Load Transient Response
VIN = 4.2V
VIN = 2.7V
50
1.56
Input Voltage (V)
VIN = 2.7V
40
(VOUT = 1.8V;
IOUT = 1A)
VIN = 3.6V
VIN = 2.7V
85°C
1.78
30
Switching Frequency vs. Temperature
VIN = 4.2V
(VOUT = 1.8V; IOUT = 1A)
1.81
20
Temperature (°C)
Output Voltage vs. Input Voltage
1.82
10
Time (100µs/div)
VIN = 4.2V
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VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
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PRODUCT DATASHEET
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SwitchRegTM
AAT2158
1.5A Low Noise Step-Down Converter
Typical Characteristics
Line Transient Response
Line Regulation
(VOUT = 1.8V; IOUT = 1A)
0.12
4.5
0.10
4.0
0.08
3.5
0.06
3.0
0.04
2.5
0.02
2.0
0.00
1.5
-0.02
1.0
-0.04
0.40
0.30
VOUT Error (%)
5.0
Output Voltage (AC coupled)
(bottom) (V)
Input Voltage
(top) (V)
(VOUT = 1.8V; IOUT = 1.5A; CFF = 100pF)
0.20
0.10
0.00
-0.10
-0.20
-0.30
-0.40
2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Time (100µs/div)
Input Voltage (V)
Heavy Load Switching Waveform
VIN = 4.2V
Enable Soft Start
VIN = 4.2V
(VIN = 3.6V;VVINOUT
= 1.8V; IOUT = 1.5A)
= 3.6V
VIN = 2.7V
EN
(2V/div)
VOUT
(1V/div)
IIN
(500mA/div)
2.6
2.0
2.4
0.0
2.2
-2.0
2.0
-4.0
1.8
-6.0
1.6
-8.0
1.4
-10.0
1.2
-12.0
1.0
Time (500ns/div)
Time (100µs/div)
Light Load Switching Waveform
Light Load Switching Waveform
40
0.6
0
0.5
-40
0.4
-80
0.3
-120
0.2
-160
0.1
-200
0.0
-240
-0.1
Output Voltage (AC coupled)
(top)(mV)
0.7
Time (5µs/div)
VIN = 3.6V
80
0.7
40
0.6
0
0.5
-40
0.4
-80
0.3
-120
0.2
-160
0.1
-200
0.0
-240
-0.1
Inductor Ripple Current
(bottom) (A)
80
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF)
Inductor Ripple Current
(bottom) (A)
Output Voltage (AC coupled)
(top)(mV)
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 0pF)
VIN = 2.7V
2158.2008.10.1.5
Inductor Ripple Current
(bottom) (A)
Output Voltage (AC Coupled)
(top) (mV)
(VIN = 3.6V; VOUT
3.6V IOUT = 1.5A)
VIN==1.8V;
VIN = 2.7V
4.0
Time (200µs/div)
VIN = 4.2V
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VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
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PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Typical Characteristics
Light Load Switching Waveform
Light Load Switching Waveform
0.6
0.5
0
-40
0.4
-80
0.3
-120
0.2
-160
0.1
-200
0.0
-240
-0.1
Output Voltage (AC coupled)
(top)(mV)
0.7
40
Time (5µs/div)
8
80
0.7
40
0.6
0
0.5
-40
0.4
-80
0.3
-120
0.2
-160
0.1
-200
0.0
-240
-0.1
Inductor Ripple Current
(bottom) (A)
80
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF)
Inductor Ripple Current
(bottom) (A)
Output Voltage (AC coupled)
(top)(mV)
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA; CFF = 100pF)
Time (500µs/div)
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AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Functional Block Diagram
VCC
VP
0.6V REF
FB
OP. AMP
CMP
DH
LOGIC
1MΩ
LX
DL
Temp.
Sensing
OSC
SGND
EN
Functional Description
The AAT2158 is a high performance 1.5A monolithic
step-down converter operating at a 1.4MHz switching
frequency. It minimizes external component size, optimizes efficiency over the complete load range, and produces reduced ripple and spectral noise. Apart from the
small bypass input capacitor, only a small L-C filter is
required at the output. Typically, a 3.3µH inductor and a
22µF ceramic capacitor are recommended for a 3.3V
output (see table of recommended 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 (plus
the DC drop of the external inductor). The device integrates extremely low RDS(ON) MOSFETs to achieve low
dropout voltage during 100% duty cycle operation. This
is advantageous in applications requiring high output
voltages (typically > 2.5V) at low input voltages.
2158.2008.10.1.5
PGND
The integrated low-loss MOSFET switches can provide
greater than 95% efficiency at full load. Light load operation maintains high efficiency, low ripple and low spectral noise even at lower currents (typically <150mA).
In battery-powered applications, as VIN decreases, the
converter dynamically adjusts the operating frequency
prior to dropout to maintain the required duty cycle and
provide accurate output regulation. Output regulation is
maintained until the dropout voltage, or minimum input
voltage, is reached. At 1.5A output load, dropout voltage
headroom is approximately 200mV.
The AAT2158 typically achieves better than ±0.5% output regulation across the input voltage and output load
range. A current limit of 2.0A (typical) protects the IC
and system components from short-circuit damage.
Typical no load quiescent current is 42µA.
Thermal protection completely disables switching when
the maximum junction temperature is detected. The
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PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Soft Start/Enable
junction over-temperature threshold is 140°C with 15°C
of hysteresis. Once an over-temperature or over-current
fault condition is removed, the output voltage automatically recovers.
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2158 into a low-power,
non-switching state. The total input current during shutdown is less than 1µA.
Peak current mode control and optimized internal compensation provide high loop bandwidth and excellent
response to input voltage and fast load transient events.
Soft start eliminates output voltage overshoot when the
enable or the input voltage is applied. Under-voltage
lockout prevents spurious start-up events.
Current Limit and
Over-Temperature Protection
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.
Control Loop
The AAT2158 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 shortcircuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor.
Thermal protection completely disables switching 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.
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
transconductance voltage error amplifier output. The
reference voltage is internally set to program the converter output voltage greater than or equal to 0.6V.
Enable
VIN+
R1
100
C1
10µF
11
10
R2
7
100K
9
C2
0.1µF
6
8
5
GND
VP
FB
VP
LX
VP
LX
EN
LX
VCC
N/C
N/C
N/C
PGND
PGND
SGND PGND
Internal bias of all circuits is controlled via the VCC
input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation.
LX
U1
AAT2158
12
Under-Voltage Lockout
C8
VOUT+
4
R3
15
14
13
L1
3.0µH
16
3
2
R4
59.0k
C3
22µF
1
VOUT(V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
R3 (kΩ )
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
137
187
267
GND
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C3 Murata 22µF 6.3V GRM21BR60J226ME39L X5R 0805
L1 Sumida CDRH5D28-3R0NC (see Table 2)
R1 and C2 are an optional noise filter for internal VCC.
R6, C4, C5-C7 are not populated
C8 100pF feed forward
Figure 1: AAT2158 Evaluation Schematic.
10
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PRODUCT DATASHEET
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SwitchRegTM
1.5A Low Noise Step-Down Converter
Component Selection
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 inductor should be set equal to the output voltage
numeric value in 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.
The 3.3µH CDRH4D28 series Sumida inductor has a
49.2mW worst case DCR and a 1.57A DC current rating.
At full 1.5A load, the inductor DC loss is 97mW which
gives less than 1.5% loss in efficiency for a 1.5A, 3.3V
output.
Input Capacitor
Select a 10µF to 22µF 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 C. The calculated value varies with input voltage and is a maximum when VIN is double the output
voltage.
CIN =
VO 
V 
· 1- O

VIN
VIN 
 VPP

- ESR · FS
 IO

VO 
V 
1
· 1 - O = for VIN = 2 · VO
VIN 
VIN 
4
CIN(MIN) =
2158.2008.10.1.5
AAT2158
1
 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:
IRMS = IO ·
VO 
V 
· 1- O
VIN 
VIN 
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 
D · (1 - D) =
0.52 =
1
2
for VIN = 2 · VO
IRMS(MAX) =
VO
IO
2

V 
· 1- O
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 AAT2158. 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 (see 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 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 form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain
measurements can also result.
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11
PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
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.
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.
Output Capacitor
The output capacitor limits the output ripple and provides holdup during large load transitions. A 10µF to
22µF 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 · ∆ILOAD
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 10µF. 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.
Adjustable Output Resistor Selection
The output voltage on the AAT2158 is programmed with
external resistors R3 and R4. To limit the bias current
required for the external feedback resistor string while
maintaining good noise immunity, the minimum suggested value for R4 is 59kW. Although a larger value will
further reduce quiescent current, it will also increase the
12
impedance of the feedback node, making it more sensitive to external noise and interference. Table 1 summarizes the resistor values for various output voltages with
R4 set to either 59kW for good noise immunity or 221kW
for reduced no load input current.
The external resistor R3, combined with an external
100pF feed forward capacitor (C8 in Figure 1), delivers
enhanced transient response for extreme pulsed load
applications and reduces ripple in light load conditions.
The addition of the feed forward capacitor typically
requires a larger output capacitor C3-C4 for stability. The
external resistors set the output voltage according to the
following equation:

R3 
VOUT = 0.6V 1 +

R4 
or
R3 =
VOUT
VREF

- 1  R4

VOUT (V)
R4 = 59kW
R3 (kW)
R4 = 221kW
R3 (kW)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.0
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
267
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
Table 1: AAT2158 Resistor Values for Various
Output Voltages.
The typical circuit shown in the AAT2158 evaluation
schematic is intended to be general purpose and suitable
for most applications. An additional example schematic
is shown in Figure 2 according to the design guidelines
for cases where transient load steps are more severe
and the restriction on output voltage deviation is more
stringent. To handle these cases some simple adjustments can be made.
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2158.2008.10.1.5
PRODUCT DATASHEET
Gill Sans MT Bold Italic
15/18, convert to outlines
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
U1
V IN
2.4V - 5.5V
R1
0
R2
100K
12
VP
LX
15
11
VP
LX
14
VP
LX
13
VCC
FB
4
10
9
EN
C1
10µF
C2
open
U1
C1
C3
C4
C8
R1-R4
L1
1.0 -1.5µH
1.2V/1.5A
R3
100K
C8
1nF
(optional)
C3
22µF
AAT2158
3
2
V OUT
L1
EN
N/C
16
3
PGND
N/C
6
2
PGND
N/C
8
1
PGND SGND
7
1
C4
22µF
R4
100K
5
AAT2158 AnalogicTech, Hi-Voltage Buck, TDFN34-16
Cap, MLC, 10µF/6.3V,X7R, 0805
Cap, MLC, 22µF/6.3V, X7R,0805
Cap, MLC, 22µF/6.3V, X7R, 0805
Cap, MLC, 1nF/6.3V, 0402
Carbon film resistor, 0402
TDK, VLS3012T-1R5N1R7, 1.5µH, I SAT = 1.9A, DCR = 68mΩ
Figure 2: AAT2158 Alternate Application Schematic.
The schematic in Figure 2 shows the configuration for
improved transient response in an application where the
output is stepped down to 1.2V. The adjustments consist
of adding an additional 22µF output capacitor C4,
increasing the value of the feed forward capacitor C8 to
1nF, and eliminating the RC filter network R1 and C2.
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down converter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
Thermal Calculations
There are three types of losses associated with the
AAT2158 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 losses is
given by:
PTOTAL =
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
PTOTAL =
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
VIN
+ (tsw · FS · IO + IQ) · VIN
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 qJA for the QFN33-16 package, which is 50°C/W.
VIN
TJ(MAX) = PTOTAL · ΘJA + TAMB
+ (tsw · FS · IO + IQ) · VIN
2158.2008.10.1.5
www.analogictech.com
13
PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Layout
3.
The suggested PCB layout for the AAT2158 is shown in
Figures 3 and 4. The following guidelines should be used
to help ensure a proper layout.
4.
1.
2.
The input capacitor (C1) should connect as closely as
possible to VP and PGND.
C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as
short as possible.
Figure 3: AAT2158 Evaluation Board
Top Side Layout.
14
5.
The feedback trace or FB pin should be separate
from any power trace and connect 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 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.
Connect unused signal pins to ground to avoid
unwanted noise coupling.
Figure 4: AAT2158 Evaluation Board
Bottom Side Layout.
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2158.2008.10.1.5
PRODUCT DATASHEET
Gill Sans MT Bold Italic
15/18, convert to outlines
SwitchRegTM
AAT2158
1.5A Low Noise Step-Down Converter
Design Example
Specifications
VO
VIN
FS
TAMB
3.3V @ 1.4A, Pulsed Load DILOAD = 1.4A
2.7V to 4.2V (3.6V nominal)
1.2MHz
85°C in QFN33-16 Package
Output Inductor
L1 = VO(µH) = 3.3µH; see Table 2.
For Sumida inductor CDRH4D28 3.3µH DCR = 49.2mW max.

VO
V 
3.3V
3.3V 

⋅ 1 - O1 =
⋅ 1 = 179mA
VIN  3.3µH ⋅ 1.2MHz
4.2V 
L1 ⋅ FS 
∆I1 =
IPK1 = IO1 +
∆I1
= 1.4A + 0.089A = 1.49A
2
PL1 = IO12 ⋅ DCR = 1.4A2 ⋅ 49.2mΩ = 96mW
Output Capacitor
VDROOP = 0.2V
COUT =
3 · ∆ILOAD
3 · 1.4A
=
= 17.5µF; use 22µF
0.2V · 1.2MHz
VDROOP · FS
IRMS(MAX) =
(VOUT) · (VIN(MAX) - VOUT)
1
3.3V · (4.2V - 3.3V)
·
= 52mArms
=
L · FS · VIN(MAX)
2 · 3 3.3µH · 1.2MHz · 4.2V
2· 3
1
·
Pesr = esr · IRMS2 = 5mΩ · (52mA)2 = 13.3µW
Input Capacitor
Input Ripple VPP = 50mV
CIN =
1
 VPP

- ESR · 4 · FS
 IO1 + IO2

IRMS(MAX) =
=
1
= 6.8µF; use 10µF
 50mV

- 5mΩ · 4 · 1.2MHz
 1.4A

IO
= 0.7Arms
2
P = esr · IRMS2 = 5mΩ · (0.7A)2 = 2.45mW
2158.2008.10.1.5
www.analogictech.com
15
PRODUCT DATASHEET
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
AAT2158 Losses
Total losses can be estimated by calculating the dropout (VIN = VO) losses where the power MOSFET RDS(ON) will be at
the maximum value. All values assume an 85°C ambient temperature and a 120°C junction temperature with the QFN
50°C/W package.
PLOSS = IO12 · RDS(ON)H = 1.4A2 · 0.16Ω = 0.31W
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 310mW = 101°C
The total losses are also investigated at the nominal lithium-ion battery voltage (3.6V). The simplified version of the
RDS(ON) losses assumes that the N-channel and P-channel RDS(ON) are equal.
PTOTAL = IO2 · RDS(ON) + [(tsw · FS · IO + IQ) · VIN]
= 1.4A2 · 152mΩ + [(5ns · 1.2MHz · 1.4A + 50µA) · 3.6V] = 328mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 328mW = 101°C
VOUT (V)
Inductance
(µH)
Part Number
Manufacturer
Size (mm)
3.3
3.3
2.5
1.8
1.5
1.2
1.0
0.8
0.6
3.0
3.3
2.2
1.8
1.8
1.2
1.0
1.0
1.0
CDRH5D28
CDRH4D28
CDRH4D28
CDRH4D28
CDRH4D28
CDRH4D28
SD3114-1.0
SD3114-1.0
SD3114-1.0
Sumida
Sumida
Sumida
Sumida
Sumida
Sumida
Cooper
Cooper
Cooper
6x6x3
5x5x3
5x5x3
5x5x3
5x5x3
5x5x3
3.1x3.1x1.45
3.1x3.1x1.45
3.1x3.1x1.45
Rated
Current (A)
IRMS (A)
ISAT
(A)
DCR (W)
2.07
2.07
2.07
24.0
49.2
31.3
27.5
27.5
23.6
0.042
0.042
0.042
2.4
1.57
2.04
2.2
2.2
2.56
1.67
1.67
1.67
Table 2: Surface Mount Inductors.
Manufacturer
Part Number
Value
Voltage
Temp. Co.
Case
Murata
Murata
GRM21BR60J106KE19
GRM21BR60J226ME39
10µF
22µF
6.3V
6.3V
X5R
X5R
0805
0805
Table 3: Surface Mount Capacitors.
16
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2158.2008.10.1.5
PRODUCT DATASHEET
Gill Sans MT Bold Italic
15/18, convert to outlines
AAT2158
SwitchRegTM
1.5A Low Noise Step-Down Converter
Ordering Information
Package
Marking1
Part Number (Tape and Reel)2
QFN33-16
XSXYY
AAT2158IVN-0.6-T1
All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means semiconductor
products that are in compliance with current RoHS standards, including the requirement that lead not exceed
0.1% by weight in homogeneous materials. For more information, please visit our website at
http://www.analogictech.com/about/quality.aspx.
Package Information3
QFN33-16
0.230 ± 0.050
Pin 1 Identification
3.000 ± 0.050
0.500 ± 0.050
Top View
5
1.250 ± 0.050
0.400 ± 0.100
Pin 1 Dot By Marking
3.000 ± 0.050
1
C0.3
13
9
1.250 ± 0.050
0.214 ± 0.036
0.900 ± 0.100
0.025 ± 0.025
Bottom View
Side View
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing
process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection.
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Phone (408) 737-4600
Fax (408) 737-4611
© Advanced Analogic Technologies, Inc.
AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual
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