LM3253
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SNVS791Q – FEBRUARY 2012 – REVISED MAY 2013
LM3253 High-Current Step-Down Converter for 2G/3G/4G RF Power Amplifiers
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FEATURES
1
•
2
•
•
•
•
•
•
•
•
•
•
High-Efficiency PFM and PWM Modes with
Internal Synchronous Rectification
Analog Bypass Function with Low Dropout
Resistance (45 mΩ typ.)
Dynamically Adjustable Output Voltage, 0.4V
to 3.6V (typ.), in PFM and PWM modes
3A Maximum Load Current in PWM Mode
2.7MHz (average) PWM Switching Frequency
Modulated Switching Frequency to Aid Rx
Band Compliance
Operates From a Single Li-ion Cell
(2.7V to 5.5V)
ACB Reduces Inductor Requirements and Size
Minimum Total Solution Size by Using Small
Footprint and Case Size Inductor and
Capacitors
16-bump Thin DSBGA Package
Current and Thermal Overload Protection
APPLICATIONS
•
•
•
•
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USB Datacards
Cellular Phones
Hand-Held Radios
RF PC Cards
Battery-Powered RF Devices
DESCRIPTION
The LM3253 is a DC-DC converter optimized for
powering multi-mode 2G/3G/4G RF power amplifiers
(PAs) from a single Lithium-Ion cell. The LM3253
steps down an input voltage from 2.7V to 5.5V to a
dynamically adjustable output voltage of 0.4V to 3.6V.
The output voltage is set through a VCON analog
input that adjusts the output voltage to ensure
efficient operation at all power levels of the RF PA.
The LM3253 is optimized for USB datacard
applications.
The LM3253 operates in constant frequency Pulse
Width Modulation (PWM) mode producing a small
and predictable amount of output voltage ripple. This
enables best ECTEL power requirements in GMSK
and EDGE spectral compliance, with the minimal
amount of filtering and excess headroom. When
operating in Pulse Frequency Modulation (PFM)
mode, the LM3253 enables the lowest DG09 current
consumption and therefore maximizes system
efficiency.
The LM3253 has a unique Active Current assist and
analog Bypass (ACB) feature to minimize inductor
size without any loss of output regulation for the
entire battery voltage and RF output power range,
until dropout. ACB provides a parallel current path,
when needed, to limit the maximum inductor current
to 1.84A (typ.) while still driving a 3A load. The ACB
also enables operation with minimal dropout voltage.
The LM3253 is available in a small 2 mm x 2 mm
chip-scale 16-bump DSBGA package.
When considering the use of the LM3253 in a system
design, contact the Texas Instruments Sales or Field
Application engineer for a copy of the "LM3253:
DC-DC Converter for 3G/4G RF PAs PCB Layout
Considerations.”
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2012–2013, Texas Instruments Incorporated
LM3253
SNVS791Q – FEBRUARY 2012 – REVISED MAY 2013
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Typical System Application Diagram
VBATT
10 µF
EN
1.5 µH
PVIN VDD
VOUT
SW
BP
10 µF
GPO1
FB
LM3253
GPO2
MODE
DAC
VCON
VCC_PA_2G
ACB
4.7 µF
BGND
PGND SGND
VCC_PA_3G
BB or
RFIC
PA
1.0 µF
PA(s)
Connection Diagram
PGND
SW
PVIN
ACB
A
A
ACB
PVIN
SW
PGND
PGND
SW
PVIN
ACB
B
B
ACB
PVIN
SW
PGND
SGND
EN
BP
BGND
C
C
BGND
BP
EN
SGND
VDD
VCON
MODE
FB
D
D
FB
MODE
VCON
VDD
1
2
3
4
4
3
2
1
Bottom View
Top View
Figure 1. 16-Bump 0.4 mm Pitch Thin DSBGA Package
2
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Pin Descriptions
Pin #
Name
Description
PGND
Power Ground to the internal NFET switch.
C1
SGND
Signal Analog and Control Ground (Low Current).
D1
VDD
Analog Supply Input.
SW
Switching Node connection to the internal PFET switch and NFET synchronous rectifier. Connect
to an inductor with a saturation current rating that exceeds the ILIM,PFET,Steady State Current Limit
specification of the LM3253.
C2
EN
Enable Input. Set this digital input HIGH for normal operation. For shutdown, set low. Pin has an
800 kΩ internal pulldown resistor.
D2
VCON
Voltage Control Analog input. VOUT = 2.5 x VCON.
PVIN
Power Supply Voltage Input to the internal PFET switch and ACB.
A1
B1
A2
B2
A3
B3
C3
BP
Bypass Mode Input. Set the pin HIGH for forced Bypass mode operation. Set the pin LOW for
automatic Analog Bypass Mode (recommended).
D3
MODE
PWM/PFM Mode Selection Input. Setting the pin HIGH allows for PFM or PWM, depending on
the load current. Setting the pin LOW forces the part to be in PWM only.
A4
B4
ACB
C4
BGND
D4
FB
Analog Current Bypass. Connect to the output at the output filter capacitor.
Active Current assist and analog Bypass Ground (High Current).
Feedback Analog Input. Connect to the output at the output filter capacitor.
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LM3253
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This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
ABSOLUTE MAXIMUM RATINGS (1) (2) (3)
VDD, PVIN to SGND
−0.2V to +6.0V
PGND to SGND
−0.2V to +0.2V
EN, FB, VCON, BP, MODE
(SGND −0.2V) to (VDD +0.2V)
SW, ACB
(PGND −0.2V) to (PVIN +0.2V
−0.2V to +0.2V
PVIN to VDD
Continuous Power Dissipation
(4)
Internally Limited
Junction Temperature (TJ-MAX)
+150°C
Storage Temperature Range
−65°C to +150°C
Maximum Lead Temperature
(Soldering, 10 sec)
+260°C
ESD Rating
(1)
(2)
(3)
(4)
(5)
(6)
4
(5) (6)
Human Body Model
2kV
Stresses beyond those listed under absolute maximum ratings 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 under recommended operating
conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
All voltages are with respect to the potential at the GND pins. The LM3253 is designed for mobile phone applications where turn-on after
power-up is controlled by the system controller and where requirements for a small package size overrule increased die size for internal
Under Voltage Lock-Out (UVLO) circuitry. Thus, it should be kept in shutdown by holding the EN pin LOW until the input voltage
exceeds 2.7V.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 150°C (typ.) and
disengages at TJ = 130°C (typ.).
The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7) The machine
model is a 200 pF capacitor discharged directly into each pin.
Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper ESD
handling procedures can result in damage.
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OPERATING RATINGS (1)
Input Voltage Range
2.7V to 5.5V
Recommended Load Current
0A to 3.0A
−30°C to +125°C
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range (2)
(1)
(2)
−30°C to +90°C
All voltages are with respect to the potential at the GND pins. The LM3253 is designed for mobile phone applications where turn-on after
power-up is controlled by the system controller and where requirements for a small package size overrule increased die size for internal
Under Voltage Lock-Out (UVLO) circuitry. Thus, it should be kept in shutdown by holding the EN pin LOW until the input voltage
exceeds 2.7V.
In applications where high-power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be de-rated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
125°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). At higher power levels duty
cycle usage is assumed to drop (i.e., max power 12.5% usage is assumed) for 2G mode.
THERMAL PROPERTIES
Junction-to-Ambient Thermal
50°C/W
Resistance (θJA), YFQ Package (1)
(1)
Junction-to-ambient thermal resistance (θJA) is taken from thermal modeling result, performed under the conditions and guidelines set
forth in the JEDEC standard JESD51-7.
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ELECTRICAL CHARACTERISTICS (1) (2) (3)
Limits in standard typeface are for TA = TJ = 25°C. Limits in boldface type apply over the full operating ambient temperature
range (−30°C ≤ TA = TJ ≤ +90°C). Unless otherwise noted, all specifications apply to the Typical System Application Diagram
with: PVIN = VDD = EN = 3.8V, BP = 0V.
Symbol
Parameter
Conditions
VFB,
LOW
Feedback voltage at low
setting
VFB,
HIGH
Feedback voltage at high
setting
VCON = 1.4V, VIN = 3.9V,
MODE = LOW (3)
Shutdown supply current
EN = SW = VCON = 0V
(4)
DC bias current into VDD
No switching (5)
MODE = HIGH
ISHDN
Iq_PFM
VCON = 0.16V, MODE = LOW
(3)
Min
Typ
Max
Units
0.3500
0.400
0.450
V
3.492
3.6
3.708
V
0.02
4
µA
260
310
975
1100
2.3
2.5
A
µA
(5)
No Switching
MODE = LOW
Iq_PWM
DC bias current into VDD
ILIM,PFET, Transient
Positive transient peak current
VCON = 0.6V (6)
limit
ILIM,PFET,Steady
Positive steady state peak
current limit
VCON = 0.6V (6)
1.78
1.9
2.09
A
ILIM, P_ACB
Positive active current assist
peak current limit
VCON = 0.6V,
VACB = 2.8V (6)
1.40
1.70
2.00
A
ILIM, NFET
NFET Switch negative peak
current limit
VCON = 1.0V (6)
−1.69
−1.50
−1.31
A
FOSC
Average internal oscillator
frequency
VCON = 1.0V
2.43
2.70
2.97
MHz
VIH
Logic HIGH input threshold
BP, EN, MODE
1.2
VIL
Logic LOW input threshold
BP, EN, MODE
IEN
EN pin pulldown current
EN = 3.6V
0
IIN
Pin input current
BP, MODE
−1
1
IVCON
VCON pin leakage current
VCON = 1.0V
−1
1
Gain
VCON to VOUT Gain
0.16V ≤ VCON ≤ 1.44V (6)
State
(1)
(2)
(3)
(4)
(5)
(6)
6
0.5
5
2.5
V
10
µA
V/V
All voltages are with respect to the potential at the GND pins. The LM3253 is designed for mobile phone applications where turn-on after
power-up is controlled by the system controller and where requirements for a small package size overrule increased die size for internal
Under Voltage Lock-Out (UVLO) circuitry. Thus, it should be kept in shutdown by holding the EN pin LOW until the input voltage
exceeds 2.7V.
Min and Max limits are specified by design, test, or statistical analysis.
The parameters in the electrical characteristics table are tested under open loop conditions at PVIN = VIN = 3.8V. For performance over
the input voltage range and closed-loop results, refer to the datasheet curves.
Shutdown current includes leakage current of PFET.
Iq specified here is when the part is not switching. For operating input current at no load, refer to datasheet curves.
Current limit is built-in, fixed, and not adjustable.
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SYSTEM CHARACTERISTICS
The following spec table entries are specified by design and verifications providing the component values in the Typical
Application Circuit are used (L = 1.5 µH, DCR = 90 mΩ, TOKO DFE252010C (1269AS-H-1R5N), CIN = 10 µF, 6.3V, 0402,
Samsung CL05A106MQ5NUN, COUT = 10 µF + 4.7 µF + 3 x 1.0 µF + 3300 pF: 6.3V, 0402, Samsung CL05A106MQ5NUN,
CL05A475MQNRN; 6.3V, 0201 Samsung CL03A105MQ3CSN; 6.3V, 01005 Murata GRM022R60J332K).
These parameters are not verified by production testing. Min and Max values are specified over the ambient temperature
range TA = −30°C to 90°C. Typical values are specified at PVIN = VDD = EN = 3.8V, BP = 0V and TA = 25°C unless
otherwise stated.
Symbol
Parameter
Conditions
Min
Typ
tSETUP
Time for SW pin to become
active upon power-up
EN = LOW-to-HIGH
tON
Turn-on time (time for output
to reach 90% of final value
after EN LOW-to-HIGH
transition)
EN = LOW-to-HIGH, VIN = 4.2V,
VCON = 1.36V, VOUT = 3.4V,
IOUT ≤ 1 mA
Time for VOUT to rise from 0V
to 3V (90% or 2.7V)
VIN = 4.2V
RLOAD = 6.8Ω,
VCON = 0V to 1.2V
Time for VOUT to fall from
3.6V to 2.6V (10% or 2.7V)
VIN = 4.2V, RLOAD = 6.8Ω,
VCON = 1.44V to 1.04V
Time for VOUT to rise from
1.8V to 2.8V (90% or 2.7V)
VIN = 4.2V, RLOAD = 1.9Ω,
VCON = 0.72V to 1.12V
Time for VOUT to fall from
2.8V to 1.8V (10% or 1.9V)
VIN = 4.2V, RLOAD = 1.9Ω,
VCON = 1.12V to 0.72V
Time for VOUT to rise from 0V
to 3.4V (90% or 3.1V)
VIN = 4.2V, RLOAD = 1.9Ω,
VCON = 0V to 1.4V
Time for VOUT to fall from
3.4V to 0.4V (10% or 0.7V)
VIN = 4.2V, RLOAD = 1.9Ω,
VCON = 1.4V to 0.16V
tBypass
Time for VOUT to rise from 0V
to PVIN after BP LOW-toHIGH transition (90%)
VCON = 0V, IOUT ≤ 1mA
tBypass, ON
Bypass turn-on time. Time for
VOUT to rise from 0V to PVIN
after EN LOW-to-HIGH
transition (90% or 3.24)
EN = VIN= 3.8V, IOUT ≤ 1 mA
Rtot_drop
Total dropout resistance in
bypass mode
VCON = 1.5V, Max value at
VIN = 3.1V,
Inductor ESR ≤ 151 mΩ
45
CIN
Pin input capacitance for BP,
EN, MODE
Test frequency = 100 KHz
5
IOUT
Maximum load current in
PWM mode
Switcher + ACB
IOUT, PU
Maximum output transient
pullup current limit
tRESPONSE
Units
30
µs
50
20
15
µs
20
20
µs
50
55
mΩ
pF
3.0
3.4
A
Switcher + ACB (1)
IOUT, PD, PWM
PWM maximum output
transient pulldown current limit
IOUT, MAX-PFM
Maximum output load current
in PFM mode
VIN = 3.8V, VCON < 1V
MODE = HIGH (1)
Linearity (2)
Linearity in control range of
VCON = 0.16V to 1.44V
VIN = 4.2V (1)
Monotonic in nature
(1)
(2)
Max
−3.0
85
mA
−3
+3
%
−50
+50
mV
Current limit is built-in, fixed, and not adjustable.
Linearity limits are ±3% or ±50 mV, whichever is larger.
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SYSTEM CHARACTERISTICS (continued)
The following spec table entries are specified by design and verifications providing the component values in the Typical
Application Circuit are used (L = 1.5 µH, DCR = 90 mΩ, TOKO DFE252010C (1269AS-H-1R5N), CIN = 10 µF, 6.3V, 0402,
Samsung CL05A106MQ5NUN, COUT = 10 µF + 4.7 µF + 3 x 1.0 µF + 3300 pF: 6.3V, 0402, Samsung CL05A106MQ5NUN,
CL05A475MQNRN; 6.3V, 0201 Samsung CL03A105MQ3CSN; 6.3V, 01005 Murata GRM022R60J332K).
These parameters are not verified by production testing. Min and Max values are specified over the ambient temperature
range TA = −30°C to 90°C. Typical values are specified at PVIN = VDD = EN = 3.8V, BP = 0V and TA = 25°C unless
otherwise stated.
Symbol
η
Parameter
Efficiency
VRIPPLE
Conditions
Min
Typ
VIN = 3.8V, VOUT = 1.8V,
IOUT = 10 mA
MODE = HIGH (PFM)
79
82
VIN = 3.8V, VOUT = 0.5V,
IOUT = 5 mA
MODE = HIGH (PFM)
58
60
VIN = 3.8V, VOUT = 3.5V,
IOUT = 1900 mA
MODE = LOW (PWM)
89
92
VIN = 3.8V, VOUT = 2.5V,
IOUT = 250 mA
MODE = LOW (PWM)
90
93
VIN = 3.8V, VOUT = 1.6V,
IOUT = 130 mA
MODE = LOW (PWM)
83
86
VIN = 3.8V, VOUT = 1V,
IOUT = 400 mA
MODE = LOW (PWM)
81
84
Max
%
Ripple voltage at no pulse
skipping condition
VIN = 3.4V to 3.6V, VOUT = 0.4V to
3.6V, ROUT = 1.9Ω (3)
MODE = LOW
Ripple voltage at pulse
skipping condition
VIN = 5.5V to dropout, VOUT = 3.6V,
ROUT = 1.9Ω (3)
8
VIN = 3.2V, VOUT < 1.125V,
IOUT =10 mA, MODE = HIGH
50
VIN = 3.2V, VOUT ≤ 0.5V,
IOUT = 5 mA, MODE = LOW
50
PFM Ripple Voltage
Units
1
3
mVpp
Line_tr
Line transient response
VIN = 3.6V to 4.2V, TR = TF = 10 µs,
VOUT = 1V, IOUT = 600 mA
MODE = LOW
50
mVpk
Load_tr
Load transient response
VOUT = 3.0V, TR = TF = 10 µs,
IOUT = 0A to 1.2A
MODE = LOW
40
mVpk
Max Duty cycle
Maximum duty cycle
MODE = LOW
100
VIN = 3.2V, VOUT = 1V, IOUT = 10 mA
MODE = HIGH
100
160
VIN = 3.2V, VOUT = 0.5V, IOUT = 5 mA
MODE = HIGH
34
55
PFM_Freq
(3)
8
Minimum PFM Frequency
%
kHz
Ripple voltage should be measured at COUT electrode on a well-designed PC board and using the suggested inductor and capacitors.
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TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs. Load Current
VIN = 3.8V, IOUT = 150mA to 750mA
100
100
95
95
90
90
EFFICENCY (%)
EFFICIENCY (%)
Efficiency vs. Load Current
VIN = 3.8V, IOUT = 10mA to 150mA
85
80
75
VOUT = 1.0V
VOUT = 1.5V
VOUT = 2.0V
VOUT = 2.5V
VOUT = 3.0V
70
65
85
80
75
VOUT = 1.6V
VOUT = 2.0V
VOUT = 2.5V
VOUT = 3.0V
VOUT = 3.5V
70
65
60
60
0
20
40 60 80 100 120 140 160
LOAD CURRENT (mA)
Figure 2.
0
Efficiency vs. Load Current
VIN = 3.8V, IOUT = 100mA to 1A
100 200 300 400 500 600 700 800
LOAD CURRENT (mA)
Figure 3.
Efficiency vs. Load Current
VIN = 5V, IOUT = 1A to 2.5A
100
100
90
80
EFFICIENCY (%)
EFFICIENCY (%)
95
90
85
80
VOUT = 1.6V
VOUT = 2.0V
VOUT = 2.5V
VOUT = 3.0V
VOUT = 3.5V
75
70
60
50
40
30
20
10
70
0
0
200
400
600
800
LOAD CURRENT (mA)
Figure 4.
1000
900
Output Voltage vs. Supply Voltage
VOUT = 3.4V, VIN = 4.3V down to dropout
3.6
Output Voltage vs. VCON Voltage
VIN = 4.2V, RLOAD = 6.8Ω, 0.16V < VCON < 1.4V
4.0
OUTPUT VOLTAGE (V)
3.2
DROPOUT
3.0
2.8
2.6
2.4
2.5
1200 1500 1800 2100 2400 2700
LOAD CURRENT (mA)
Figure 5.
3.5
3.4
OUTPUT VOLTAGE (V)
VOUT = 2.0V
VOUT = 2.5V
VOUT = 3.0V
VOUT = 3.5V
3.0
2.5
2.0
1.5
1.0
0.5
IOUT= 1.5A
2.5X GAIN
0.0
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE (V)
Figure 6.
5.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
VCON (V)
Figure 7.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Quiescent Current (PFM) vs. Supply Voltage
VOUT = 1V, 2.7V < VIN< 5.5V (No Load)
290
2.95
QUIESCENT CURRENT ( A)
SWITCHING FREQUENCY (MHz)
Center-Switching Frequency vs. Supply Voltage
VOUT = 2.5V, IOUT = 700mA, VIN = 3.8V
3.00
2.90
2.85
2.80
2.75
2.70
2.65
2.60
2.55
2.50
3.0
270
260
250
240
230
220
210
3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
Figure 8.
6.0
2.5
Quiescent Current (PWM) vs. Supply Voltage
VOUT = 2.5V, 2.7V < VIN< 5.5V (No Load)
12
QUIESCENT CURRENT (mA)
280
3.0
3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
Figure 9.
5.5
6.0
VCON Transient (3G/4G)
VOUT = 0V to 3V, RLOAD = 6.8Ω, VIN = 3.8V
10
8
VOUT
2V/DIV
VCON
2V/DIV
6
4
2
IOUT
500 mA/DIV
0
2.5
3.0
3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
Figure 10.
5.5
20 s/DIV
6.0
Figure 11.
VCON Transient (2G)
VOUT = 1.4V to 3.4V, RLOAD = 1.9Ω, VIN = 4.2V
VOUT
2V/DIV
VCON
2V/DIV
Load Transient in PFM Mode
VOUT = 1V, IOUT = 0mA to 60mA, VIN = 3.6V
VOUT
5 mV/DIV
IOUT
50 mA/DIV
1A/DIV
IOUT
20 s/DIV
20 s/DIV
Figure 12.
10
Figure 13.
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SNVS791Q – FEBRUARY 2012 – REVISED MAY 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN
Load Transient in PWM Mode
= 3.8V, VOUT = 2.5V, IOUT = 0mA to 300mA
VOUT
20 mV/
DIV
IOUT
200 mA/
DIV
Load Transient in PWM Mode
VIN = 3.8V, VOUT = 3V, IOUT = 0mA to 700mA
VOUT
50 mV/
DIV
IOUT
500 mA/
DIV
100 Ps/DIV
100 Ps/DIV
Figure 14.
Figure 15.
Load Transient in PWM Mode
VIN = 4.2V, VOUT = 3V, IOUT = 0mA to 1.2A
Line Transient
VIN = 3.6V to 4.2V, VOUT = 2.5V, RLOAD = 6.8Ω
100 mV/
DIV
VOUT
VOUT
50 mV/DIV
1V/DIV
VIN
500 mA/
DIV
IOUT
100 s/DIV
100 Ps/DIV
Figure 16.
Figure 17.
Line Transient
VIN = 3.6V to 4.2V, VOUT = 1V, RLOAD = 6.8Ω
Startup in PFM Mode
VIN = 3.8V, VOUT = 1V, No Load, EN = LOW to HIGH
50 mV/DIV
VOUT
2V/DIV
VSW
1V/DIV
VOUT
VIN
1V/DIV
2V/DIV
EN
20 s/DIV
100 s/DIV
Figure 18.
Figure 19.
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LM3253
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN
Startup in PWM Mode
= 4.2V, VOUT = 3.4V, No Load, EN = LOW to HIGH
Timed-Current Limit
VIN = 4.2V, VOUT = 2.5V, RLOAD = 6.8Ω to VOUT Shorted
VOUT
2V/DIV
VSW
2V/DIV
2V/DIV
VSW
2V/DIV
Inductor
Current
1A/DIV
VOUT
2V/DIV
EN
20 s/DIV
40 s/DIV
Figure 20.
12
Figure 21.
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LM3253
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SNVS791Q – FEBRUARY 2012 – REVISED MAY 2013
OPERATION DESCRIPTION
Device Information
The LM3253 is a high-efficiency step-down DC-DC converter optimized to power the RF power amplifier (PA) in
cell phones, portable communication devices, or battery-powered RF devices with a single Li-Ion battery. It
operates in fixed-frequency PWM mode for 2G transmissions (with MODE = LOW), automatic mode transition
between PFM and PWM mode for 3G/4G RF PA operation (with MODE = HIGH), forced bypass mode (with BP
= HIGH) or in shutdown mode (with EN = LOW).
The fixed-frequency Pulse Width Modulation (PWM) mode provides high efficiency and very low output voltage
ripple. In Pulse Frequency Modulation (PFM) mode, the converter operates with reduced switching frequencies
and lower supply current to maintain high efficiencies. The forced bypass mode allows the user to drive the
output directly from the input supply through a bypass FET. The shutdown mode turns the LM3253 off and
reduces current consumption to 0.02 µA (typ.).
In PWM and PFM modes of operation, the output voltage of the LM3253 can be dynamically programmed from
0.4V to 3.6V (typ.) by adjusting the voltage on VCON. Current overload protection and thermal overload
protection are also provided.
The LM3253 was engineered with Active Current assist and analog Bypass (ACB). This unique feature allows
the converter to support maximum load currents of 3A (min.) while keeping a small footprint inductor and meeting
all of the transient behaviors required for operation of a multi-mode RF Power Amplifier. The ACB circuit provides
an additional current path when the load current exceeds 1.9A (typ.) or as the switcher approaches dropout.
Similarly, the ACB circuit allows the converter to respond with faster VCON output voltage transition times by
providing extra output current on rising and falling output edges. The ACB circuit also performs the function of
analog bypass. Depending upon the input voltage, output voltage and load current, the ACB circuit automatically
and seamlessly transitions the converter into analog bypass while maintaining output voltage regulation and low
output voltage ripple. Full bypass (100% duty cycle operation) will occur if the total dropout resistance in bypass
mode (Rtot_drop = 45 mΩ) is insufficient to regulate the output voltage.
The LM3253 16-bump DSBGA package is the best solution for space-constrained applications such as cell
phones and other hand-held devices. The high switching frequency, 2.7 MHz (typ.) in PWM mode, reduces the
size of input capacitors, output capacitors and of the inductor. Use of a DSBGA package is best suited for
opaque case applications and requires special design considerations for implementation. (Refer to DSBGA
Package Assembly And Use section below). As the LM3253 does not implement UVLO, the system controller
should set EN = LOW and set BP = HIGH during power-up and UVLO conditions. (Refer to Shutdown Mode
below).
PWM Operation
When the LM3253 operates in PWM mode, the switching frequency is constant, and the switcher regulates the
output voltage by changing the energy-per-cycle to support the load required. During the first portion of each
switching cycle, the control block in the LM3253 turns on the internal PFET switch. This allows current to flow
from the input through the inductor and to the output filter capacitor and load. The inductor limits the current to a
ramp with a slope of (VIN – VOUT)/L, by storing energy in its magnetic field.
During the second portion of each cycle, the control block turns the PFET switch off, blocking current flow from
the input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the
NFET and to the output filter capacitor and load, which ramps the inductor current down with a slope of –VOUT/L.
The output filter capacitor stores charge when the inductor current is greater than the load current and releases it
when the inductor current is less than the load current, smoothing the voltage across the load.
At the next rising edge of the clock, the cycle repeats. An increase of load pulls the output voltage down,
increasing the error signal. As the error signal increases, the peak inductor current becomes higher, thus
increasing the average inductor current. The output voltage is therefore regulated by modulating the PFET switch
on-time to control the average current sent to the load. The circuit generates a duty-cycle modulated rectangular
signal that is averaged using a low pass filter formed by the inductor and output capacitor. The output voltage is
equal to the average of the duty-cycle modulated rectangular signal.
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PFM Mode
With MODE = HIGH, the LM3253 automatically transitions to from PWM into PFM operation if the average
inductor current is less than 75 mA (typ.) and VIN − VOUT > 0.6V. The switcher regulates the fixed output voltage
by transferring a fixed amount of energy during each cycle and modulating the frequency to control the total
power delivered to the output. The converter switches only as needed to support the demand of the load current,
therefore maximizing efficiency. If the load current should increase during PFM mode to more than 95 mA (typ.),
the part will automatically transition into constant frequency PWM mode. A 20 mA (typ.) hysteresis window exists
between PFM and PWM transitions.
After a transient event, the part temporarily operates in 2.7 MHz (typ.) fixed-frequency PWM mode to quickly
charge or discharge the output. This is true for start-up conditions or if MODE pin is toggled LOW-to-HIGH. Once
the output reaches its target output voltage, and the load is less than 75 mA (typ.), then the part will seamlessly
transition into PFM mode (assuming it is not in forced bypass or auto bypass condition).
Active Current Assist and Analog Bypass (ACB)
The 3GPP time mask requirement for 2G requires high current to be sourced by the LM3253. These high
currents are required for a small time during transients or under a heavy load. Over-rating the switching inductor
for these higher currents would increase the solution size and will not be an optimum solution. So to allow an
optimal inductor size for such a load, an alternate current path is provided from the input supply through the ACB
pin. Once the switcher current limit ILIM,PFET,SteadyState is reached, the ACB circuit starts providing the additional
current required to support the load. The ACB circuit also minimizes the dropout voltage by having the analog
bypass FET in parallel with VOUT. The LM3253 can provide up to 3A (min.) of current in bypass mode with a 4A
(max.) peak current limit.
Bypass Operation
The Bypass Circuit provides an analog bypass function with very low dropout resistance (Rtot_drop = 45 mΩ typ).
When BP = LOW the part will be in automatic bypass mode which will automatically determine the amount of
bypass needed to maintain voltage regulation. When the input supply voltage to the LM3253 is lowered to a level
where the commanded duty cycle is higher than what the converter is capable of providing, the part will go into
pulse-skipping mode. The switching frequency will be reduced to maintain a low and well-behaved output voltage
ripple. The analog bypass circuit will allow the converter to stay in regulation until full bypass is reached (100%
duty cycle operation). The converter comes out of full bypass and back into analog bypass regulation mode with
a similar reverse process.
To override the automatic bypass mode, either set VCON > (VIN)/(2.5) (but less than VIN) or set BP = HIGH for
forced bypass function. Forced bypass function is valid for 2.7V < VIN < 5.5V.
Shutdown Mode
To shut down the LM3253, pull the EN pin LOW (1.2V), and the mode of operation will be dependent on the
voltage applied to the MODE pin.
Since the LM3253 does not feature a UVLO (Under Voltage Lock-Out) circuit, the EN pin should be set LOW to
turn off the LM3253 during power-up and during UVLO conditions. For cell-phone applications, the system
controller determines the power supply sequence; thus, it is up to the system controller to ensure proper
sequencing by using all of the available pins and functions properly.
Mode Pin
The MODE pin changes the state of the converter to one of the two allowed modes of operation. Setting the
MODE pin HIGH (>1.2V) sets the device for automatic transition between PFM/PWM mode operation. In this
mode, the converter operates in PFM mode to maintain the output voltage regulation at very light loads and
transitions into PWM mode at loads exceeding 95 mA (typ.). The PWM switching frequency is 2.7 MHz (typ.).
Setting the MODE pin LOW (
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