NSC LM3350

LM3350
Switched Capacitor Voltage Converter
General Description
The LM3350 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 LM3350 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.
n
n
n
n
Features
n Conversion of 3.3V to 5V, or 5V to 3.3V
n Small Mini SO-8 package
n No inductor required
Applications
n
n
n
n
Key Specifications
n 800 kHz switch frequency allows use of very small,
inexpensive capacitors.
4.2Ω typical step-up output impedance
1.8Ω typical step-down output impedance
90% typical power conversion efficiency at 50 mA
250 nA typical shutdown current
Any mixed 5V and 3.3V system
Laptop computers and PDAs
Handheld instrumentation
PCMCIA cards
Ordering Information
Order Number
Package Type
NSC Package
Drawing
Package
Marking
LM3350MMX
Mini SO-8
MUA08A
S00A
3500k Units on Tape and Reel
LM3350MM
Mini SO-8
MUA08A
S00A
1000 Units on Tape and Reel
Supplied As
Basic Operating Circuits
Step-Up Converter
Step-Down Converter
DS100002-1
DS100002-2
Connection Diagram
Mini SO8 Package
DS100002-3
Top View
© 1999 National Semiconductor Corporation
DS100002
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LM3350 Switched Capacitor Voltage Converter
December 1999
LM3350
Absolute Maximum Ratings (Note 1)
Storage Temperature
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Lead Temperature (Soldering, 10
secs)
Maximum Input Voltage, Step-Down
3.65V
Power Dissipation (PD) (TA=25˚C,
(Note 2))
500 mW
θja (Note 2)
250˚C/W
260˚C
ESD Susceptibility (Note 3)
2kV
Not short circuit protected.
5.5V
Maximum Input Voltage, Step-Up
−65˚C to +150˚C
Operating Conditions (Note 1)
Ambient Temperature Range
Tj Max (Note 2)
−40˚C to + 85˚C
150˚C
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.
Symbol
Parameter
Conditions
Typ (Note 4)
Limits (Note 5)
Units
VoutNL
Output Voltage at
No Load
C1, C2, Cin and Cout: 0.33µF, esr ≤
0.33 Ω at 1MHz. Iload = 0 A
5.0
V
VoutFL
Output Voltage at
50 mA
C1, C2, Cin and Cout: 0.33µF, esr ≤
0.33 Ω at 1MHz. 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
3.75
Output Source
Impedance
Iload = 50 mA
4.2
Switching
Frequency
(Note 6)
Quiescent Current
V(Min)
3.65
V(Max)
3
µA(Max)
5
mA(Max)
6.25
Ω (Max)
500
kHz(Min)
1100
kHz(Max)
1.0
V(Min)
2.5
V(Max)
1
µA(Max)
µA
mA
Ω
800
Enable Threshold
Voltage
Leakage Current
V
2.5
kHz
1.7
Current into ENABLE pin; ENABLE =
5V and all other pins at ground
Power Efficiency
V
0.025
µA
%
90
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.
Symbol
Parameter
Conditions
Typ (Note 4)
Limits (Note 5)
Units
VoutNL
Output Voltage at
No Load
C1, C2, Cin and Cout: 0.33µF, esr ≤
0.33 Ω at 1MHz. Iload = 0 A
3.3
V
VoutFL
Output Voltage at
50 mA
C1, C2, Cin and Cout: 0.33 µF, esr ≤
0.33Ω at 1MHz. Iload =50 mA
3.2
V
Vin
Input Supply
Voltage Range
IQ1
Quiescent Current
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5
Shutdown Mode, VEnable = 0V,
Iload=0 A; Current into pin Vhigh
2
V
2.2
V(Min)
5.5
V(Max)
3
µA(Max)
0.25
µA
LM3350
Electrical Characteristics
2/3 Step-Down Voltage Converter
(Continued)
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.
Symbol
IQ2
Zout
fSW
VEnable
IEnable
Pη
Parameter
Conditions
Typ (Note 4)
Normal Mode, IIoad = 0A; Current into
pin Vhigh
2.5
Output Source
Impedance
Iload = 50 mA
1.8
Switching
Frequency
(Note 6)
Quiescent Current
Units
3.3
mA(Max)
3
Ω (Max)
500
kHz(Min)
1100
kHz(Max)
1.0
V(Min)
2.5
V(Max)
1
µA(Max)
mA
Ω
800
Enable Threshold
Voltage
Leakage Current
Limits (Note 5)
kHz
1.7
Current into ENABLE pin; ENABLE =
5V and all other pins at ground
V
0.025
Power Efficiency
µA
%
90
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, LM3350 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 half of the oscillator frequency, fOSC = 2fSW.
Typical Performance Characteristics
Vout vs Iload (Step-Up)
Pη vs Iload (Step-Up)
DS100002-4
Output Source Impedance vs
Temperature (Step-Up)
DS100002-6
DS100002-9
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LM3350
Typical Performance Characteristics
Vout vs Iload (Step-Down)
(Continued)
Pη vs Iload (Step-Down)
DS100002-5
Output Source Impedance vs
Temperature (Step-Down)
DS100002-7
DS100002-8
Switching Frequency vs
Temperature
DS100002-10
Pin Description
Detailed Operation
Pin
Name
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
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Function
OPERATING PRINCIPLE
The LM3350 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 LM3350 contains an array of CMOS switches which are
operated in a certain sequence to provide the step-up or
step-down 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.
Active high CMOS logic level Enable
Input. Connect to Voltage Input terminal
to enable the IC. Connect to Ground
(Pin 6) to disable.
4
LM3350
Detailed Operation
(Continued)
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3350 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 2). 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 3).
SHUTDOWN MODE
When ENABLE is a logic low (ground), the LM3350 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 5.0V for step-down mode), the LM3350 returns to normal operation.
DS100002-11
A. Step-Up Mode
Application Information
CAPACITOR SELECTION
The LM3350 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 2, 3). 0.33 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 input bypass capacitor (Cin) is the least critical. Its value
should be at least that of the sampling capacitors.
While the recommended sampling capacitor values are 0.33
µF, other values may be selected. Values other than 0.33 µF
affect maximum output current rating and efficiency. Figure 1
shows typical output impedance (Zout) versus capacitor
value.
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 (∆Vout) requirements from:
DS100002-12
B. Step-Down Mode
FIGURE 1. Zout vs. C
PRECAUTIONS
The LM3350 is not short circuit protected.
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.
5
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LM3350
Typical Application Circuits
DS100002-13
FIGURE 2. Step-Up Converter
DS100002-14
FIGURE 3. Step-Down Converter
Layout Information
LAYOUT CONSIDERATIONS
The LM3350’s high switching frequency (800 kHz) makes a good layout important. Figure 4 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 and high frequency ringing. Of the four capacitors, Chigh and Clow have the highest
value of di/dt. It is therefore most important to keep them close to the IC. The ground lead that Chigh and Clow 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.
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6
LM3350
Layout Information
(Continued)
DS100002-16
B. Actual Size of the Layout
DS100002-15
A. Copper side (5X)
DS100002-17
C. Copper side with Component Locations (5X)
FIGURE 4. Typical Layout
7
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LM3350 Switched Capacitor Voltage Converter
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead Mini SO (MM)
Order Number LM3350MMX or LM3350MM
NS Package Number MUA08A
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