LM3351
LM3351 Switched Capacitor Voltage Converter
Literature Number: SNIS104B
LM3351
Switched Capacitor Voltage Converter
General Description
Features
The LM3351 is a CMOS charge-pump voltage converter
which efficiently provides a 3.3V to 5V step-up, or 5V to 3.3V
step-down. The LM3351 is pin for pin compatible with the
LM3350 but consumes 66% less quiescent current. The
LM3351 uses four small, low cost capacitors to provide the
voltage conversion. It eliminates the cost, size and radiated
EMI related to inductor based circuits, or the power loss of a
linear regulator. Operating power conversion efficiency
greater than 90% provides ideal performance for battery powered portable systems.
The architecture provides a fixed voltage conversion ratio of
3/2 or 2/3. Thus it can be used for other DC-DC conversions
as well.
■ Conversion of 3.3V to 5V, or 5V to 3.3V
■ 200 kHz switch frequency allows use of very small,
■
■
■
■
■
■
■
inexpensive capacitors.
4.2Ω typical step-up output impedance
1.8Ω typical step-down output impedance
95% typical power conversion efficiency at 50 mA
250 nA typical shutdown current
No inductor required
Low quiescent current extends battary life
Small Mini SO-8 package
Applications
■
■
■
■
Any mixed 5V and 3.3V system
Laptop computers and PDAs
Handheld instrumentation
PCMCIA cards
Basic Operating Circuits
Step-Up Converter
10014601
Step-Down Converter
10014602
© 2011 National Semiconductor Corporation
100146
100146 Version 6 Revision 3
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Print Date/Time: 2011/09/22 14:50:05
LM3351 Switched Capacitor Voltage Converter
OBSOLETE
September 22, 2011
LM3351
Connection Diagram
Mini SO8 Package
10014603
Top View
Ordering Information
Order Number
Package Type
NSC Package
Drawing
Package Marking
LM3351MMX
Mini SO-8
MUA08A
S05A
3500 Units on Tape and Reel
LM3351MM
Mini SO-8
MUA08A
S05A
1000 Units on Tape and Reel
Supplied As
Pin Descriptions
Pin
Name
Function
1
Cap1+ Positive terminal for the first charge pump
capacitor.
2
Cap1− Negative terminal for the first charge pump
capacitor.
3
Cap2+ Positive terminal for the second charge
pump capacitor.
4
Cap2− Negative terminal for the second charge
pump capacitor.
5
Vlow
In Step-Up mode, this will be the input
terminal. In Step-Down mode, this will be
the output terminal.
6
Gnd
Ground
7
Vhigh
In Step-Down mode, this will be the input
terminal. In Step-Up mode, this will be the
output terminal.
8
Enable Active high CMOS logic level Enable Input.
Connect to Voltage Input terminal to
enable the IC. Connect to Ground (Pin 6)
to disable.
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Operating Conditions
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Input Voltage, Step-Down
Maximum Input Voltage, Step-Up
Power Dissipation (PD) (TA=25°C,
(Note 2))
5.5V
3.65V
500 mW
θja (Note 2)
Tj Max (Note 2)
Storage Temperature
Lead Temperature (Soldering, 10 secs)
ESD Susceptibility (Note 3)
Not short circuit protected.
250°C/W
Ambient Temperature Range
(Note 1)
−40°C to + 85°C
150°C
−65°C to +150°C
260°C
2kV
Electrical Characteristics
3/2 Step-Up Voltage Converter
Specifications in standard type face are for Tj = 25°C, and those with boldface type apply over full operating temperature
range. Unless otherwise specified, Vin = 3.3V, VEnable = 3.3V, Iload = 50 mA, C1, C2, Cin and Cout = 1 µF.
Symbol
Parameter
Typ (Note 4)
Conditions
Limits (Note 5)
Units
VoutNL
Output Voltage at No
Load
Iload = 0 A
5.0
V
VoutFL
Output Voltage at 50
mA
Iload = 50 mA
4.7
V
Vin
Input Supply Voltage
Range
IQ1
IQ2
Zout
fSW
VEnable
IEnable
Pη
Quiescent Current
3.3
Shutdown Mode, VEnable = 0V, Iload=0 A;
Current into pin Vlow
0.025
Normal Mode, IIoad = 0A; Current into pin
VIow
1.1
Output Source
Impedance
Iload = 50 mA
4.2
Switching Frequency
(Note 6)
Quiescent Current
V
V(Min)
V(Max)
3
µA
µA(Max)
1.5
mA
mA(Max)
Ω
6.25
Ω (Max)
125
275
kHz
kHz(Min)
kHz(Max)
1.0
2.5
V
V(Min)
V(Max)
1
µA
µA(Max)
200
Enable Threshold
Voltage
Leakage Current
2.5
3.65
1.7
Current into ENABLE pin; ENABLE = 5V
and all other pins at ground
Power Efficiency
0.025
95
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%
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LM3351
Absolute Maximum Ratings (Note 1)
LM3351
Electrical Characteristics
2/3 Step-Down Voltage Converter
Specifications in standard type face are for Tj = 25°C, and those with boldface type apply over full operating temperature
range. Unless otherwise specified, Vhigh = 5V, VEnable = 5V, Iload = 50 mA, C1, C2, Cin and Cout = 1 µF.
Symbol
Parameter
Typ (Note 4)
Conditions
Limits (Note 5)
Units
VoutNL
Output Voltage at No
Load
Iload = 0 A
3.3
V
VoutFL
Output Voltage at 50
mA
Iload =50 mA
3.2
V
Vin
Input Supply Voltage
Range
IQ1
IQ2
Zout
fSW
VEnable
IEnable
Pη
Quiescent Current
5
Shutdown Mode, VEnable = 0V, Iload=0 A;
Current into pin Vhigh
0.25
Normal Mode, IIoad = 0A; Current into pin
Vhigh
0.8
Output Source
Impedance
Iload = 50 mA
1.8
Switching Frequency
(Note 6)
Quiescent Current
V
V(Min)
V(Max)
3
µA
µA(Max)
1.0
mA
mA(Max)
3
Ω (Max)
125
275
kHz
kHz(Min)
kHz(Max)
1.0
2.5
V
V(Min)
V(Max)
1
µA
µA(Max)
Ω
200
Enable Threshold
Voltage
Leakage Current
2.2
5.5
1.7
Current into ENABLE pin; ENABLE = 5V
and all other pins at ground
Power Efficiency
0.025
95
%
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see Electrical Characteristics. The
guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the
listed test conditions.
Note 2: For operation at elevated temperatures, LM3351 must be derated based on package thermal resistance of θja and Tj max, Tj = TA+ θjaPD.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kW resistor into each pin.
Note 4: Typical numbers are at 25°C and represent the most likely parametric norm.
Note 5: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality
Control (SQC) methods. The limits are used to calculate National's Averaging Outgoing Quality Level (AOQL).
Note 6: The internal oscillator runs at 1.6 MHz, the output switches operate at one eighth of the oscillator frequency, fOSC = 8fSW.
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LM3351
Typical Performance Characteristics
Vout vs Iload (Step-Up)
Pη vs Iload (Step-Up)
10014604
10014605
Output Source Impedance vs Temperature (Step-Up)
Vout vs Iload (Step-Down)
10014606
10014607
Output Source Impedance vs Temperature (Step-Down)
Pη vs Iload (Step-Down)
10014609
10014608
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LM3351
Switching Frequency vs Temperature
10014610
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OPERATING PRINCIPLE
The LM3351 is a charge-pump voltage converter that provides a voltage conversion ratio of 3/2 in step-up mode and a
conversion ratio of 2/3 in the step-down mode. Thus it can be
used in the step-down mode to provide a 3.3V output from a
regulated 5V input or in the step-up mode to provide a 5V
output from a regulated 3.3V input. Other values of input voltages can be used as long as they are within the limits.
The LM3351 contains an array of CMOS switches which are
operated in a certain sequence to provide the step-up or stepdown of the input supply. An internal RC oscillator provides
the timing signals.
Energy transfer and storage are provided by four inexpensive
ceramic capacitors. The selection of these capacitors is explained in the Capacitor Selection section under Application
Information.
Application Information
CAPACITOR SELECTION
The LM3351 requires four capacitors: an input bypass capacitor (connected between Vin and ground), an output hold
capacitor (connected between Vout and ground), and two
sampling capacitors (C1 and C2 in Figures 1, 2). 1.0 µF
(±20%) ceramic chip type capacitors are recommended for all
four capacitors. The usable operating frequency should be
greater than 5 MHz for all capacitors.
The output hold capacitor value determines the output ripple.
Increasing the value of the hold capacitor decreases the ripple. The value of this capacitor (Cout) can be calculated (approximately) based on the output ripple (ΔVout) requirements
from:
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3351 is biased from pin Vhigh. Thus for step-up applications, an external Schottky diode (D1) is needed to supply
power to Vhigh during startup (See Figure 1). Note that during
shutdown, this diode will provide a DC path from Vin to Vout.
The load may therefore continue to draw current from the input voltage source. This Schottky diode is not required for
step-down applications (See Figure 2).
where Iload is the load current and fO is the oscillator frequency.
In order to ensure superior performance over the entire operating temperature range, capacitors made of X7R dielectric
material are suggested. However, capacitors made of other
dielectric materials that still meet the ±20% specification over
the entire temperature range can also be used.
SHUTDOWN MODE
When ENABLE is a logic low (ground), the LM3351 enters a
low power shutdown mode. In this mode, all circuitry is disabled and therefore, all switching action stops. During shutdown, the current consumption drops to 250 nA (typical).
When ENABLE is a logic high, (i.e. 3.3V for step-up mode and
PRECAUTIONS
The LM3351 is not short circuit protected.
Typical Application Circuits
10014612
FIGURE 1. Step-Up Converter
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LM3351
5.0V for step-down mode), the LM3351 returns to normal operation.
Detailed Operation
LM3351
10014613
FIGURE 2. Step-Down Converter
and high frequency ringing. Of the four capacitors, CIN and
COUT have the highest value of di/dt. It is therefore most important to keep them close to the IC. The ground lead that
CIN and COUT share should also be kept wide and short.
The location of the diode (D1) used in the step-up configuration is not critical. This diode is only used during the initial turn
on of the IC. D1 is not needed in step-down applications.
Layout Information
LAYOUT CONSIDERATIONS
The LM3351's high switching frequency (200 kHz) makes a
good layout important. Figure 3 illustrates a typical layout. It
is important to keep the distance short between the four capacitors and the IC. Wide traces and grounding are also
recommended. These steps will minimize trace inductance
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LM3351
10014615
B. Actual Size of the Layout
10014614
A. Copper side (5X)
10014616
C. Copper side with Component Locations (5X)
FIGURE 3. Typical Layout
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LM3351
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead Mini SO (MM)
Order Number LM3351MMX or LM3351MM
NS Package Number MUA08A
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LM3351
Notes
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LM3351 Switched Capacitor Voltage Converter
Notes
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