LM3205
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SNVS388E – JULY 2005 – REVISED APRIL 2013
LM3205 650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power
Amplifiers
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FEATURES
DESCRIPTION
•
•
The LM3205 is a DC-DC converter optimized for
powering RF power amplifiers (PAs) from a single
Lithium-Ion cell, however they may be used in many
other applications. It steps down an input voltage
from 2.7V to 5.5V to a variable output voltage from
0.8V(typ.) to 3.6V(typ.). Output voltage is set using a
VCON analog input for controlling power levels and
efficiency of the RF PA.
1
2
•
•
•
•
•
•
•
•
2 MHz (typ.) PWM Switching Frequency
Operates from a Single Li-Ion Cell (2.7V to
5.5V)
Variable Output Voltage (0.8V to 3.6V)
Fast Output Voltage Transient (0.8V to 3.6V in
20µs)
650mA Maximum Load Capability
High Efficiency (96% Typ at 4.2VIN, 3.4VOUT at
400mA) from Internal Synchronous
Rectification
Current Overload Protection
Thermal Overload Protection
Packages
– 8-Pin DSBGA (Lead Free)
10-Pin WSON
The LM3205 offers superior performance for mobile
phones and similar RF PA applications. Fixedfrequency PWM operation minimizes RF interference.
Shutdown function turns the device off and reduces
battery consumption to 0.01 µA (typ.).
The LM3205 is available in DSBGA package and
WSON package. For all other package options
contact your local TI sales office.
A high switching frequency (2 MHz) allows use of tiny
surface-mount components. Only three small external
surface-mount components, an inductor and two
ceramic capacitors are required.
APPLICATIONS
•
•
•
•
Cellular Phones
Hand-Held Radios
RF PC Cards
Battery Powered RF Devices
TYPICAL APPLICATION
VIN
2.7V to 5.5V
VOUT
PVIN
VDD
0.8V to 3.6V
SW
EN
10 PF
3.3 PH
LM3205
VOUT = 2.5 x VCON
FB
4.7 PF
VCON
PGND
SGND
Figure 1. LM3205 Typical Application
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 © 2005–2013, Texas Instruments Incorporated
LM3205
SNVS388E – JULY 2005 – REVISED APRIL 2013
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CONNECTION DIAGRAMS
SW
SW
A2
A2
PVIN A1
A3 PGND
PGND
A3
A1
VDD B1
B3 SGND
SGND
B3
B1
VDD
FB C3
C1
EN
C3
EN C1
FB
PVIN
C2
C2
VCON
VCON
Top View
Bottom View
Figure 2. 8–Bump Thin DSBGA Package, Large Bump
NS Package Number YZR0008GNA
PGND
1
10
SW
SW
10
1
PGND
PGND
2
9
PVin
PVin
9
2
PGND
SGND
3
8
PVin
PVin
8
3
SGND
VCON
4
7
VDD
VDD
7
4
VCON
FB
5
6
EN
EN
6
5
FB
LM3205
Top View
LM3205
Bottom View
Figure 3. 10–Pin WSON
NS Package Number DSC0010A
PIN DESCRIPTIONS
Pin #
Name
Description
DSBGA
WSON
A1
8, 9
PVIN
Power Supply Voltage Input to the internal PFET switch.
B1
7
VDD
Analog Supply Input.
C1
6
EN
Enable Input. Set this digital input high for normal operation. For shutdown, set low.
C2
4
VCON
C3
5
FB
B3
3
SGND
Analog and Control Ground
A3
1, 2
PGND
Power Ground
A2
10
SW
Voltage Control Analog input. VCON controls VOUT in PWM mode.
Feedback Analog Input. Connect to the output at the output filter capacitor.
Switch node connection to the internal PFET switch and NFET synchronous rectifier.
Connect to an inductor with a saturation current rating that exceeds the maximum Switch Peak
Current Limit specification of the LM3205.
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
2
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ABSOLUTE MAXIMUM RATINGS (1) (2) (3)
−0.2V to +6.0V
VDD, PVIN to SGND
−0.2V to +0.2V
PGND to SGND
(SGND −0.2V) to (VDD +0.2V) w/6.0V max
EN, FB, VCON
(PGND −0.2V) to (PVIN +0.2V) w/6.0V max
SW
−0.2V to +0.2V
PVIN to VDD
Continuous Power Dissipation (4)
Internally Limited
Junction Temperature (TJ-MAX)
+150°C
−65°C to +150°C
Storage Temperature Range
Maximum Lead Temperature (Soldering, 10 sec)
ESD Rating (5) (6)
+260°C
Human Body Model
2 kV
Machine Model
(1)
(2)
(3)
(4)
(5)
(6)
200V
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is ensured. Operating Ratings do not imply specified performance limits. For specified performance limits
and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pins. The LM3205 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 TI 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 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7) The machine
model is a 200pF capacitor discharged directly into each pin.
TI recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper ESD handling techniques
can result in damage.
OPERATING RATINGS (1) (2)
Input Voltage Range
2.7V to 5.5V
Recommended Load Current
0mA to 650mA
−30°C to +125°C
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range (3)
(1)
(2)
(3)
−30°C to +85°C
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is ensured. Operating Ratings do not imply specified performance limits. For specified performance limits
and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pins. The LM3205 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).
THERMAL PROPERTIES
Junction-to-Ambient Thermal DSBGA
100°C/W
Resistance (θJA), DSBGA YZR08 Package (1)
Junction-to-Ambient Thermal WSON
Resistance (θJA), WSON DSC0010A Package
(1)
55°C/W
(1)
DSBGA: Junction-to-ambient thermal resistance (θJA) is taken from thermal measurements, performed under the conditions and
guidelines set forth in the JEDEC standard JESD51-7. A 4 layer, 4" x 4", 2/1/1/2 oz. Cu board as per JEDEC standards is used for the
measurements. WSON: The value of (θJA) in WSON-10 could fall in a range of 50°C/W to 150°C/W (if not wider), depending on PWB
material, layout, and environmental conditions. In applications where high maximum power dissipation exits (high VIN , high IOUT ),
special care must be paid to thermal dissipation areas. For more information on these topics for WSON, refer to
Application Note 1187: Leadless Leadframe Package (LLP) (SNOA401) and the Power Efficiency and Power Dissipation section
of this datasheet
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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 ≤ +85°C). Unless otherwise noted, all specifications apply to LM3205TL/LM3205SD with: PVIN = VDD
= EN = 3.6V.
Symbol
Parameter
Conditions
VFB,
MIN
Feedback Voltage at
minimum setting
VCON = 0.32V (3)
VFB,
MAX
Feedback Voltage at
maximum setting
VCON = 1.44V, VIN = 4.2V (3)
Min
Typ
Max
Units
0.75
0.8
0.85
V
3.537
3.6
3.683
V
(4)
ISHDN
Shutdown supply current
EN = SW = VCON = 0V
IQ
DC bias current into VDD
VCON = 2V, FB = 0V, No Switching (5)
0.01
2
µA
1
1.4
mA
RDSON(P) DSBGA
Pin-pin resistance for PFET ISW = 200mA
140
200
230
mΩ
RDSON(N) DSBGA
Pin-pin resistance for NFET ISW = -200mA
300
415
485
mΩ
RDSON(P)WSON
Pin-pin resistance for PFET ISW = 200mA
170
230
260
mΩ
RDSON(N)WSON
Pin-pin resistance for NFET ISW = -200mA
330
445
515
mΩ
ILIM,PFET
Switch peak current limit
935
1100
1200
mA
FOSC
Internal oscillator frequency
1.7
2
2.3
MHz
VIH,EN
Logic high input threshold
1.2
VIL,EN
Logic low input threshold
IPIN,EN
Pin pull down current
ZCON
VCON input resistance
Gain
VCON to VOUT Gain
(1)
(2)
(3)
(4)
(5)
(6)
4
See (6)
V
5
100
0.32V ≤ VCON ≤ 1.44V
0.5
V
10
µA
kΩ
2.5
V/V
All voltages are with respect to the potential at the GND pins. The LM3205 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. Typical numbers are not ensured, but do represent the most
likely norm. Due to the pulsed nature of the testing TA = TJ for the electrical characteristics table.
The parameters in the electrical characteristics table are tested under open loop conditions at PVIN = VDD = 3.6V. 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 operating at 100% duty cycle.
Current limit is built-in, fixed, and not adjustable. Refer to datasheet curves for closed loop data and its variation with regards to supply
voltage and temperature. Electrical Characteristic table reflects open loop data (FB = 0V and current drawn from SW pin ramped up until
cycle by cycle limit is activated). Closed loop current limit is the peak inductor current measured in the application circuit by increasing
output current until output voltage drops by 10%.
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SYSTEM CHARACTERISTICS
The following spec table entries are specified by design providing the component values in the typical application circuit are
used. These parameters are not specified by production testing. Min and Max limits apply over the full operating ambient
temperature range (−30°C ≤ TA ≤ 85°C) and over the VIN range = 2.7V to 5.5V, TA = 25°C, PVIN = VDD = EN = 3.6V, L =
3.3µH, DCR of L ≤ 100mΩ, CIN = 10µF, 0603, 6.3V (4.7µF||4.7µF, 0603, 6.3V can be used), COUT = 4.7µF, 0603, 6.3V for
LM3205TL/LM3205SD unless otherwise noted.
Symbol
TRESPONSE
Parameter
Typ
Max
Units
VIN = 4.2V, COUT = 4.7µF, L = 3.3µH,
RLOAD = 5.5Ω
20
30
µs
Time for VOUT to fall from 3.6V to VIN = 4.2V, COUT = 4.7µF, L = 3.3µH,
0.8V
RLOAD = 10Ω
20
30
µs
20
pF
-3
+3
%
-10
10
µA
100
µs
Time for VOUT to rise from 0.8V
to 3.6V
Conditions
Min
CCON
VCON input capacitance
VCON = 1V,
Test frequency = 100 kHz
Linearity
Linearity in control
range 0.32V to 1.44V
VIN = 3.9V
Monotonic in nature
ICON
Control pin input current
TON
Turn on time
(time for output to reach 3.6V
from Enable low to high
transition)
EN = Low to High, VIN = 4.2V, VO = 3.6V,
COUT = 4.7µF, IOUT ≤ 1mA
70
Efficiency
(L = 3.3µH, DCR ≤ 100mΩ)
VIN = 3.6V, VOUT = 0.8V, IOUT = 90mA
83
%
VIN = 4.2V, VOUT = 3.4V, IOUT = 400mA
96
%
VOUT_ripple Ripple voltage, PWM mode
VIN = 3V to 4.5V, VOUT = 0.8V, IOUT =
10mA to 400mA (1)
10
mVp-p
Line_tr
VIN = 600mV perturbance,
TRISE = TFALL = 10µs, VOUT = 0.8V, IOUT =
100mA
50
mVpk
η
Line transient response
Load_tr
Load transient response
VIN = 3.1/3.6/4.5V, VOUT = 0.8V, transients
up to 100mA, TRISE = TFALL = 10µs
50
mVpk
PSRR
VIN = 3.6V, VOUT = 0.8V, IOUT =
100mA
sine wave perturbation
frequency = 10kHz, amplitude = 100mVp-p
40
dB
(1)
Ripple voltage should measured at COUT electrode on good layout PC board and under condition using suggested inductors and
capacitors.
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TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit in Figure 31, PVIN = VDD = EN = 3.6V, L = 3.3uH, DCR of L ≤ 100mΩ, CIN = 10uF, 0603, 6.3V ( 4.7uF||4.7uF, 0603,
6.3V can be used), COUT = 4.7uF, 0603, 6.3V for LM3205TL/LM3205SD unless otherwise noted)
Quiescent Current
vs
Supply Voltage
(VCON = 2V, FB = 0V, No Switching)
Shutdown Current
vs
Temperature
(VCON = 0V, EN = 0V)
1.4
QUIESCENT CURRENT (mA)
1.3
TA = 85oC
1.2
TA = 25oC
1.1
1.0
TA = -30oC
0.9
0.8
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
SUPPLY VOLTAGE (V)
Figure 4.
Figure 5.
Switching Frequency Variation
vs
Temperature
(VOUT = 1.3V, IOUT = 200mA)
Output Voltage
vs
Supply Voltage
(VOUT = 1.3V)
SWITCHING FREQUENCY VARIATION (%)
4.0
3.0
VIN = 5.5V
2.0
VIN = 4.2V
1.0
0.0
-1.0
VIN = 3.6V
-2.0
VIN = 2.7V
-3.0
-4.0
-40
-20
0
20
40
60
80
100
AMBIENT TEMPERATURE (oC)
Figure 6.
(VIN
Figure 7.
Output Voltage
vs
Temperature
= 3.6V, VOUT = 0.8V)
(VIN
Figure 8.
6
Output Voltage
vs
Temperature
= 3.6V, VOUT = 3.4V)
Figure 9.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
(Circuit in Figure 31, PVIN = VDD = EN = 3.6V, L = 3.3uH, DCR of L ≤ 100mΩ, CIN = 10uF, 0603, 6.3V ( 4.7uF||4.7uF, 0603,
6.3V can be used), COUT = 4.7uF, 0603, 6.3V for LM3205TL/LM3205SD unless otherwise noted)
Open/Closed Loop Current Limit
vs
Temperature
(PWM mode)
VCON Voltage
vs
Output Voltage
(VIN = 4.2V, RLOAD = 8Ω)
Figure 10.
Figure 11.
Efficiency
vs
Output Voltage
(VIN = 3.9V)
Efficiency
vs
Output Current
(VOUT = 0.8V)
Figure 12.
Figure 13.
Efficiency
vs
Output Current
(VOUT = 3.4V)
Load Transient Response
(VOUT = 0.8V)
50 mV/DIV
AC Coupled
VOUT
VIN = 3.6V
VOUT = 0.8V
IL
200 mA/DIV
250 mA
IOUT
50 mA
10 Ps/DIV
Figure 14.
Figure 15.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
(Circuit in Figure 31, PVIN = VDD = EN = 3.6V, L = 3.3uH, DCR of L ≤ 100mΩ, CIN = 10uF, 0603, 6.3V ( 4.7uF||4.7uF, 0603,
6.3V can be used), COUT = 4.7uF, 0603, 6.3V for LM3205TL/LM3205SD unless otherwise noted)
Load Transient Response
(VIN = 4.2V, VOUT = 3.4V)
Startup
(VIN = 3.6V, VOUT = 1.3V, RLOAD = 1kΩ)
100 mV/DIV
AC Coupled
VOUT
VIN = 4.2V
VOUT = 3.4V
IL
200 mA/DIV
400 mA
IOUT
100 mA
10 Ps/DIV
Figure 16.
Figure 17.
Startup
(VIN = 4.2V, VOUT = 3.4V, RLOAD = 5kΩ)
Shutdown Response
(VIN = 4.2V, VOUT = 3.4V, RLOAD = 10Ω)
VSW
5V/DIV
VIN = 4.2V
VOUT
VOUT = 3.4V
RL = 10:
2V/DIV
IL
500 mA/DIV
2V/DIV
EN
40 Ps/DIV
Figure 18.
Figure 19.
Line Transient Response
(VIN = 3.0V to 3.6V, IOUT = 100mA)
VCON Voltage Response
(VIN = 4.2V, VCON = 0.32V/1.44V, RLOAD = 10Ω)
VSW
2V/DIV
3.6V
VOUT
VIN = 4.2V
VCON = 0.32/1.44V
RL = 10:
0.8V
1.44V
VCON
0.32V
40 Ps/DIV
Figure 20.
8
Figure 21.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
(Circuit in Figure 31, PVIN = VDD = EN = 3.6V, L = 3.3uH, DCR of L ≤ 100mΩ, CIN = 10uF, 0603, 6.3V ( 4.7uF||4.7uF, 0603,
6.3V can be used), COUT = 4.7uF, 0603, 6.3V for LM3205TL/LM3205SD unless otherwise noted)
VCON and Load Transient
(VIN = 4.2V, VCON = 0.32V/1.44V, 15Ω/8Ω, same time)
Timed Current Limit Response
(VIN = 3.6V)
2V/DIV
VSW
3.6V
VOUT
VIN = 4.2V
VCON = 0.32/1.44V
RL = 15:/8:
0.8V
1.44V
VCON
0.32V
40 Ps/DIV
Figure 22.
Figure 23.
Output Voltage Ripple
(VOUT = 1.3V)
Output Voltage Ripple
(VOUT = 3.4V)
VSW
5V/DIV
VIN = 4.2V
VOUT = 3.4V
IOUT = 200 mA
VOUT
10 mV/DIV
AC Coupled
IL
100 mA/DIV
200 ns/DIV
Figure 24.
Figure 25.
Output Voltage Ripple in Pulse Skip
(VIN = 3.64V, VOUT = 3.4V, RLOAD = 5Ω)
RDSON
vs
Temperature (DSBGA)
(P-ch, ISW = 200mA)
VSW
2V/DIV
10 mV/DIV
AC Coupled
VOUT
VIN = 3.64V
VOUT = 3.4V
RL = 5 :
500 mA/DIV
IL
400 ns/DIV
Figure 26.
Figure 27.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
(Circuit in Figure 31, PVIN = VDD = EN = 3.6V, L = 3.3uH, DCR of L ≤ 100mΩ, CIN = 10uF, 0603, 6.3V ( 4.7uF||4.7uF, 0603,
6.3V can be used), COUT = 4.7uF, 0603, 6.3V for LM3205TL/LM3205SD unless otherwise noted)
10
RDSON
vs
Temperature (DSBGA)
(N-ch, ISW = -200mA)
EN High Threshold
vs.
Vin
Figure 28.
Figure 29.
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BLOCK DIAGRAM
PVIN
VDD
CURRENT
SENSE
OSCILLATOR
FB
ERROR
AMPLIFIER
~
PWM
COMP
MOSFET
CONTROL
LOGIC
VCON
SW
MAIN CONTROL
EN
SHUTDOWN
CONTROL
SGND
PGND
Figure 30. Functional Block Diagram
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OPERATION DESCRIPTION
The LM3205 is a simple, step-down DC-DC converter optimized for powering RF power amplifiers (PAs) in
mobile phones, portable communicators, and similar battery powered RF devices. It is designed to allow the RF
PA to operate at maximum efficiency over a wide range of power levels from a single Li-Ion battery cell. It is
based on a current-mode buck architecture, with synchronous rectification for high efficiency. It is designed for a
maximum load capability of 650mA in PWM mode.
Maximum load range may vary from this depending on input voltage, output voltage and the inductor chosen.
Efficiency is typically around 96% for a 400mA load with 3.4V output, 4.2V input. The output voltage is
dynamically programmable from 0.8V (typ.) to 3.6V (typ.) by adjusting the voltage on the control pin without the
need for external feedback resistors. This ensures longer battery life by being able to change the PA supply
voltage dynamically depending on its transmitting power.
Additional features include current overload protection and thermal overload shutdown.
The LM3205 is constructed using a chip-scale 8-pin DSBGA or 10-pin WSON package. These packages offers
the smallest possible size, for space-critical applications such as cell phones, where board area is an important
design consideration. Use of a high switching frequency (2MHz) reduces the size of external components. As
shown in Figure 1, only three external power components are required for implementation. Use of a DSBGA
package requires special design considerations for implementation. (See DSBGA Package Assembly and use in
the Applications Information section.) Its fine bump-pitch requires careful board design and precision assembly
equipment. Use of this package is best suited for opaque-case applications, where its edges are not subject to
high-intensity ambient red or infrared light. Also, the system controller should set EN low during power-up and
other low supply voltage conditions. (See Shutdown Mode in the Device Information section.)
VIN
2.7V to 5.5V
C1*
10 PF
PVIN
VDD
L1
3.3 PH
VOUT
0.8V to 3.6V
SW
SYSTEM
DAC
CONTROLLER
ON/OFF
VCON
LM3205
FB
C2
4.7 PF
EN
SGND
PGND
* Place C1 close to PVIN
Figure 31. Typical Operating System Circuit
CIRCUIT OPERATION
Referring to Figure 1 and Figure 30, the LM3205 operates as follows. During the first part of each switching
cycle, the control block in the LM3205 turns on the internal PFET (P-channel MOSFET) switch. This allows
current to flow from the input through the inductor to the output filter capacitor and load. The inductor limits the
current to a ramp with a slope of around (VIN - VOUT) / L, by storing energy in a magnetic field. During the second
part of each cycle, the controller turns the PFET switch off, blocking current flow from the input, and then turns
the NFET (N-channel MOSFET) synchronous rectifier on. In response, the inductor’s magnetic field collapses,
generating a voltage that forces current from ground through the synchronous rectifier to the output filter
capacitor and load. As the stored energy is transferred back into the circuit and depleted, the inductor current
ramps down with a slope around VOUT / L. The output filter capacitor stores charge when the inductor current is
high, and releases it when low, smoothing the voltage across the load.
The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the
load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and
synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output
voltage is equal to the average voltage at the SW pin.
12
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While in operation, the output voltage is regulated by switching at a constant frequency and then modulating the
energy per cycle to control power to the load. Energy per cycle is set by modulating the PFET switch on-time
pulse width to control the peak inductor current. This is done by comparing the signal from the current-sense
amplifier with a slope compensated error signal from the voltage-feedback error amplifier. At the beginning of
each cycle, the clock turns on the PFET switch, causing the inductor current to ramp up. When the current sense
signal ramps past the error amplifier signal, the PWM comparator turns off the PFET switch and turns on the
NFET synchronous rectifier, ending the first part of the cycle. If an increase in load pulls the output down, the
error amplifier output increases, which allows the inductor current to ramp higher before the comparator turns off
the PFET. This increases the average current sent to the output and adjusts for the increase in the load.
Before appearing at the PWM comparator, a slope compensation ramp from the oscillator is subtracted from the
error signal for stability of the current feedback loop. The minimum on time of PFET is 50ns (typ.)
SHUTDOWN MODE
Setting the EN digital pin low (1.2V) enables normal operation.
EN should be set low to turn off the LM3205 during power-up and under voltage conditions when the power
supply is less than the 2.7V minimum operating voltage. The LM3205 is designed for compact portable
applications, such as mobile phones. In such applications, the system controller determines power supply
sequencing and requirements for small package size outweigh the additional size required for inclusion of UVLO
(Under Voltage Lock-Out) circuitry.
INTERNAL SYNCHRONOUS RECTIFICATION
While in PWM mode, the LM3205 uses an internal NFET as a synchronous rectifier to reduce rectifier forward
voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compared to the voltage drop across and ordinary rectifier
diode.
With medium and heavy loads, the internal NFET synchronous rectifier is turned on during the inductor current
down slope in the second part of each cycle. The synchronous rectifier is turned off prior to the next cycle. The
NFET is designed to conduct through its intrinsic body diode during transient intervals before it turns on,
eliminating the need for an external diode.
CURRENT LIMITING
A current limit feature allows the LM3205 to protect itself and external components during overload conditions. In
PWM mode, an 1200mA (max.) cycle-by-cycle current limit is normally used. If an excessive load pulls the output
voltage down to approximately 0.375V, then the device switches to a timed current limit mode. In timed current
limit mode the internal PFET switch is turned off after the current comparator trips and the beginning of the next
cycle is inhibited for 3.5us to force the instantaneous inductor current to ramp down to a safe value. The
synchronous rectifier is off in timed current limit mode. Timed current limit prevents the loss of current control
seen in some products when the output voltage is pulled low in serious overload conditions.
DYNAMICALLY ADJUSTABLE OUTPUT VOLTAGE
The LM3205 features dynamically adjustable output voltage to eliminate the need for external feedback resistors.
The output can be set from 0.8V(typ.) to 3.6V(typ.) by changing the voltage on the analog VCON pin. This feature
is useful in PA applications where peak power is needed only when the handset is far away from the base station
or when data is being transmitted. In other instances the transmitting power can be reduced. Hence the supply
voltage to the PA can be reduced, promoting longer battery life. See Setting the Output Voltage in the Application
Information section for further details.
THERMAL OVERLOAD PROTECTION
The LM3205 has a thermal overload protection function that operates to protect itself from short-term misuse and
overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both
the PFET and the NFET are turned off in PWM mode. When the temperature drops below 125°C, normal
operation resumes. Prolonged operation in thermal overload conditions may damage the device and is
considered bad practice.
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APPLICATION INFORMATION
SETTING THE OUTPUT VOLTAGE
The LM3205 features a pin-controlled variable output voltage to eliminate the need for external feedback
resistors. It can be programmed for an output voltage from 0.8V (typ.) to 3.6V (typ.) by setting the voltage on the
VCON pin, as in the following formula:
VOUT = 2.5 x VCON
(1)
When VCON is between 0.32V and 1.44V, the output voltage will follow proportionally by 2.5 times of VCON.
If VCON is over 1.44V (VOUT = 3.6V), sub-harmonic oscillation may occur because of insufficient slope
compensation. If VCON voltage is less than 0.32V (VOUT = 0.8V), the output voltage may not be regulated due to
the required on-time being less than the minimum on-time (50ns). The output voltage can go lower than 0.8V
providing a limited VIN range is used. Refer to datasheet curve (VCON Voltage vs Output Voltage) for details. This
curve is for a typical part and there could be part-to-part variation for output voltages less than 0.8V over the
limited VIN range.
INDUCTOR SELECTION
A 3.3µH inductor with saturation current rating over 1200mA and low inductance drop at the full DC bias
condition is recommended for almost all applications. The inductor’s DC resistance should be less than 0.2Ω for
good efficiency. For low dropout voltage, lower DCR inductors are advantageous. The lower limit of acceptable
inductance is 2.3µH at 1200mA over the operating temperature range. Full attention should be paid to this limit,
because some small inductors show large inductance drops at high DC bias. These can not be used with the
LM3205. Taiyo-Yuden NR3015T3R3M is an example of an inductor with the lowest acceptable limit (as of
Nov./05). Table 1 suggests some inductors and suppliers.
Table 1. Suggested inductors and their suppliers
Model
Size (WxLxH) [mm]
Vendor
NR3015T3R3M
3.0 x 3.0 x 1.5
Taiyo-Yuden
DO3314-332MXC
3.3 x 3.3 x 1.4
Coilcraft
If a smaller inductance inductor is used in the application, the LM3205 may become unstable during line and load
transients and VCON transient response times may get affected.
For low-cost applications, an unshielded bobbin inductor is suggested. For noise-critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is to lay out the board with footprints accommodating
both types for design flexibility. This allows substitution of a low-noise toroidal inductor, in the event that noise
from low-cost bobbin models is unacceptable. Saturation occurs when the magnetic flux density from current
through the windings of the inductor exceeds what the inductor’s core material can support with a corresponding
magnetic field. This can cause poor efficiency, regulation errors or stress to a DC-DC converter like the LM3205.
CAPACITOR SELECTION
The LM3205 is designed for use with ceramic capacitors for its input and output filters. Use a 10µF ceramic
capacitor for input and a 4.7µF ceramic capacitor for output. They should maintain at least 50% capacitance at
DC bias and temperature conditions. Ceramic capacitors types such as X5R, X7R are recommended for both
filters. These provide an optimal balance between small size, cost, reliability and performance for cell phones
and similar applications. Table 2 lists some suggested part numbers and suppliers. DC bias characteristics of the
capacitors must be considered when selecting the voltage rating and case size of the capacitor. A few
manufactures can supply 4.7µF capacitors in the 0805 case size which maintain at least 50% of their value, but
TDK is currently the only manufacturer which can provide such capacitors in the 0603 case size. As of
November, 2005, no manufacture can supply 10µF capacitors in the 0603 case size which maintain 50% of their
value. If it is necessary to choose a 0603-size capacitor for VIN, the operation of the LM3205 should be carefully
evaluated on the system board. Output capacitors with smaller case sizes mitigate piezo electric vibrations when
the output voltage is stepped up and down at fast rates. However, they have a larger percentage drop in value
with dc bias. Use of multiple 2.2µF or 1µF capacitors in parallel may also be considered.
14
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Table 2. Suggested capacitors and their suppliers
Model
Vendor
0805ZD475KA 4.7µF, 10V
AVX
C1608X5R0J475M, 4.7µF, 6.3V
TDK
C2012X5R0J106M,10µF, 6.3V
TDK
The input filter capacitor supplies AC current drawn by the PFET switch of the LM3205 in the first part of each
cycle and reduces the voltage ripple imposed on the input power source. The output filter capacitor absorbs the
AC inductor current, helps maintain a steady output voltage during transient load changes and reduces output
voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low ESR
(Equivalent Series Resistance) to perform these functions. The ESR of the filter capacitors is generally a major
factor in voltage ripple.
EN PIN CONTROL
Drive the EN pin using the system controller to turn the LM3205 ON and OFF. Use a comparator, Schmidt trigger
or logic gate to drive the EN pin. Set EN high (>1.2V) for normal operation and low (
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