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LM2750
Low Noise, 5.0V Regulated Switched Capacitor Voltage
Converter
General Description
The LM2750 is a switched-capacitor doubler that produces a
low-noise, well regulated, 5.0V output. It can supply up to
120mA of output current over a 2.9V to 5.6V input range, as
well as up to 40mA of output current when the input voltage
is as low as 2.7V. The LM2750 has been placed in National’s
10-pin LLP, a package with excellent thermal properties that
keeps the part from overheating under almost all rated operating conditions
A perfect fit for space-constrained, battery-powered applications, the LM2750 requires only 3 external components: one
input capacitor, one output capacitor, and one flying capacitor. Small, inexpensive ceramic capacitors are recommended for use. These capacitors, in conjunction with the
1.7MHz fixed switching frequency of the LM2750, yield low
output voltage ripple, beneficial for systems requiring a lownoise 5V supply. Pre-regulation minimizes input current
ripple, reducing input noise to negligible levels.
A tightly controlled soft-start feature limits inrush currents
during part activation. Shutdown completely disconnects the
load from the input. Output current limiting and thermal
shutdown circuitry protect both the LM2750 and connected
devices in the event of output shorts or excessive current
loads.
Features
n Inductorless solution: Application requires only 3 small
ceramic capacitors
n Low Noise, 5.0V
±
4% Regulated Output
n 85% Peak Efficiency
70% Average Efficiency over Li-Ion Input Range
(2.9V-to-4.2V)
n Output Current up to 120mA with 2.9V ≤ V
IN
≤ 5.6V
Output Current up to 40mA with 2.7V ≤ V
IN
≤ 2.9V
n Wide Input Voltage Range: 2.7V to 5.6V
n Fixed 1.7MHz switching frequency for a low-noise,
low-ripple output signal
n Pre-regulation minimizes input current ripple, keeping
the battery line (V
IN
) virtually noise-free
n Tiny LLP package with outstanding power dissipation:
Usually no derating required.
n Shutdown Supply Current less than 2µA
Applications
n White and Colored LED-based Display Lighting
n Cellular Phone SIM cards
n Audio Amplifier Power Supplies
n General Purpose Li-Ion-to-5V Conversion
Typical Application Circuit
20035101
February 2003
LM2750 Low Noise, 5.0V Regulated Switched Capacitor Voltage Converter
© 2003 National Semiconductor Corporation DS200351 www.national.com
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Connection Diagram
LM2750
10-Pin Leadless Leadframe Package (LLP) - 3mm X 3mm
NS Package Number LDA10A
20035102
Top View
Pin Description
Pin #(s) Pin Name Description
8, 9 V
IN
Input Voltage - The pins must be connected externally.
1, 2 V
OUT
Output Voltage - These pins must be connected externally.
10 CAP+ Flying Capacitor Positive Terminal
7 CAP- Flying Capacitor Negative Terminal
4SD
Active-Low Shutdown Input. A 200kΩ resistor is connected internally between
this pin and GND to pull the voltage on this pin to 0V, and shut down the part,
when the pin is left floating.
3, 5, 6, DAP GND Ground - These pins must be connected externally.
Ordering Information
Output Voltage
Option
Ordering
Information
Package Marking Supplied as
5.0 LM2750LD-5.0 S002B 1000 Units, Tape and Reel
5.0 LM2750LDX-5.0 S002B 4500 Units, Tape and Reel
LM2750
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Absolute Maximum Ratings (Notes 1,
2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
Pin: Voltage to Ground −0.3V to 6V
SD Pin: Voltage to GND
−0.3V to
(V
IN
+0.3V)
Junction Temperature (T
J-MAX-ABS
) 150˚C
Continuous Power Dissipation Internally Limited
(Note 3)
Storage Temperature Range −65˚C to 150˚C
Maximum Lead Temperature 260˚C
(Soldering, 5 sec.)
ESD Rating (Note 4)
Human-body model:
Machine model
2kV
100V
Operating Ratings (Notes 1, 2)
Input Voltage Range 2.7V to 5.6V
Recommended Output Current
2.9V ≤ V
IN
≤ 5.6V 0 to 120mA
2.7V ≤ V
IN
≤ 2.9V 0 to 40mA
Junction Temperature (T
J
) Range -40˚C to 125˚C
Ambient Temperature (T
A
) Range -40˚C to 85˚C
(Note 5)
Thermal Information
Junction-to-Ambient Thermal
Resistance, LLP-10 55˚C/W
Package (θ
JA
) (Note 6)
Electrical Characteristics (Notes 2, 7)
Typical values and limits in standard typeface apply for TJ=25oC. Limits in boldface type apply over the operating junction
temperature range. Unless otherwise specified: 2.9V ≤ V
IN
≤ 5.6V, V(SD) = VIN,C
FLY
= 1µF, CIN= 2 x 1µF, C
OUT
=2x1µF
(Note 8).
Symbol Parameter Conditions Min Typ Max Units
V
OUT
Output Voltage
2.9V ≤ V
IN
≤ 5.6V,
I
OUT
≤ 120mA
4.80
(-4%)
5.0 5.20
(+4%)
V
(%)
2.7V ≤ V
IN
≤ 2.9V,
I
OUT
≤ 40mA
4.80
(-4%)
5.0 5.20
(+4%)
I
Q
Operating Supply Current I
OUT
= 0mA,
V
IH(MIN)
≤ V(SD) ≤V
IN
51012mA
I
SD
Shutdown Supply Current V(SD) = 0V 2 µA
V
R
Output Ripple C
OUT
= 10µF, I
OUT
= 100mA 4
mVp-p
C
OUT
=2.2µF, I
OUT
= 100mA 15
E
PEAK
Peak Efficiency VIN= 2.7V, I
OUT
= 40mA 87 %
V
IN
= 2.9V, I
OUT
= 120mA 85
E
AVG
Average Efficiency over Li-Ion
Input Range (Note 10)
VINRange: 2.9V - 4.2V,
I
OUT
= 120mA
70 %
V
IN
Range: 2.9V - 4.2V,
I
OUT
= 40mA
67
f
SW
Switching Frequency 1.0 1.7 MHz
t
ON
V
OUT
Turn-On Time VIN= 3.0V, I
OUT
= 100mA,
(Note 9)
0.5 ms
I
LIM
Current Limit V
OUT
shorted to GND 300 mA
Shutdown Pin (SD) Characteristics
V
IH
Logic-High SD Input 1.3 V
IN
V
V
IL
Logic-Low SD Input 0 0.4 V
I
IH
SD Input Current (Note 11) 1.3V ≤ V(SD) ≤ V
IN
15 50 µA
I
IL
SD Input Current V(SD) = 0V −1 1 µA
Capacitor Requirements
C
IN
Required Input
Capacitance(Note 12)
I
OUT
≤ 60mA 1.0 µF
60mA ≤ I
OUT
≤ 120mA 2.0
C
OUT
Required Output
Capacitance(Note 12)
I
OUT
≤ 60mA 1.0 µF
60mA ≤ I
OUT
≤ 120mA 2.0
LM2750
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Electrical Characteristics (Notes 2, 7) (Continued)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of
the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the
Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: Thermal shutdown circuitry protects the device from permanent damage.
Note 4: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200pF capacitor discharged
directly into each pin.
Note 5: Maximum ambient temperature (T
A-MAX
) is dependent on the maximum operating junction temperature (T
J-MAX-OP
= 125oC), the maximum power
dissipation of the device in the application (P
D-MAX
), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the
following equation: T
A-MAX=TJ-MAX-OP
-(θJAxP
D-MAX
). Maximum power dissipation of the LM2750 in a given application can be approximated using the following
equation: P
D-MAX
=(V
IN-MAXxIIN-MAX
)-(V
OUTxIOUT-MAX
)=[V
IN-MAX
x((2xI
OUT-MAX
) + 5mA)] - (V
OUTxIOUT-MAX
). In this equation, V
IN-MAX,IIN-MAX
, and
I
OUT-MAX
are the maximum voltage/current of the specific application, and not necessarily the maximum rating of the LM2750.
The maximum ambient temperature rating of 85
o
C is determined under the following application conditions: θJA=55oC/W, P
D-MAX
= 727mW (achieved when
V
IN-MAX
= 5.5V and I
OUT-MAX
= 115mA, for example). Maximum ambient temperature must be derated by 1.1oC for every increase in internal power dissipation of
20mW above 727mW (again assuming that θ
JA
=55oC/W in the application). For more information on these topics, please refer to Application Note 1187: Leadless
Leadframe Package (LLP) and the Power Efficiency and Power Dissipation section of this datasheet.
Note 6: Junction-to-ambient thermal resistance (θ
JA
) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC
standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102mm x 76mm x 1.6mm witha2x1array of thermal vias. The ground plane on the board
is 50mm x 50mm. Thickness of copper layers are 36mm/18 mm /18 mm /36 mm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22˚C, still air. Power
dissipation is 1W.
The value of θ
JA
of the LM2750 in LLP-10 could fall in a range as wide as 50oC/W to 150oC/W (if not wider), depending on PCB material, layout, and environmental
conditions. In applications where high maximum power dissipation exists (high V
IN
, high I
OUT
), special care must be paid to thermal dissipation issues. For more
information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power Dissipation
section of this datasheet. and the following sections of this datasheet:
Note 7: All room temperature limits are 100% tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed by correlation
using standard Statistical Quality Control methods (SQC). All limits are used to calculate Average Outgoing Quality Level (AOQL). Typical numbers are not
guaranteed, but do represent the most likely norm.
Note 8: C
FLY,CIN
, and C
OUT
: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
Note 9: Turn-on time is measured from when SD signal is pulled high until the output voltage crosses 90% of its final value.
Note 10: Efficiency is measured versus VIN, with VINbeing swept in small increments from 3.0V to 4.2V. The average is calculated from these measurements
results. Weighting to account for battery voltage discharge characteristics (V
BAT
vs. Time) is not done in computing the average.
Note 11: SD Input Current (I
IH
) is due to a 200kΩ (typ.) pull-down resistor connected internally between the SD pin and GND.
Note 12: Limit is the minimum required output capacitance to ensure proper operation. This electrical specification is guaranteed by design.
Block Diagram
20035103
LM2750
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Typical Performance Characteristics Unless otherwise specified: V
IN
= 3.6V, TA=25oC, CIN=
2.2µF, C
FLY
= 1.0µF, C
OUT
= 2.2µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s).
Output Voltage vs. Output Current Output Voltage vs. Output Current
20035115 20035116
Output Voltage vs. Input Voltage Power Efficiency
20035117 20035118
Input Current vs. Output Current Quiescent Supply Current
20035119
20035120
LM2750
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Typical Performance Characteristics Unless otherwise specified: V
IN
= 3.6V, TA=25oC, CIN=
2.2µF, C
FLY
= 1.0µF, C
OUT
= 2.2µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued)
Current Limit Behavior Switching Frequency
20035121
20035122
Output Voltage Ripple Output Voltage Ripple
20035112
20035113
Turn-on Behavior
20035114
LM2750
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Operation Description
OVERVIEW
The LM2750 is a regulated switched capacitor doubler that,
by combining the principles of a switched-capacitor voltage
doubler and a linear regulator, generates a regulated 5V
output from an extended Li-Ion input voltage range. A twophase non-overlapping clock generated internally controls
the operation of the doubler. During the charge phase (φ1),
the flying capacitor (C
FLY
) is connected between the input
and ground through internal pass-transistor switches and is
charged to the input voltage. In the pump phase that follows
(φ2), the flying capacitor is connected between the input and
output through similar switches. Stacked atop the input, the
charge of the flying capacitor boosts the output voltage and
supplies the load current.
A traditional switched capacitor doubler operating in this
manner will use switches with very low on-resistance, ideally
0Ω, to generate an output voltage that is 2x the input voltage.
The LM2750 regulates the output voltage by controlling the
resistance of the two input-connected pass-transistor
switches in the doubler.
PRE-REGULATION
The very low input current ripple of the LM2750, resulting
from internal pre-regulation, adds very little noise to the input
line. The core of the LM2750 is very similar to that of a basic
switched capacitor doubler: it is composed of four switches
and a flying capacitor (external). Regulation is achieved by
modulating the on-resistance of the two switches connected
to the input pin (one switch in each phase). The regulation is
done before the voltage doubling, giving rise to the term
"pre-regulation". It is pre-regulation that eliminates most of
the input current ripple that is a typical and undesirable
characteristic of a many switched capacitor converters.
INPUT, OUTPUT, AND GROUND CONNECTIONS
Making good input, output, and ground connections is essential to achieve optimal LM2750 performance. The two
input pads, pads 8 and 9, must be connected externally. It is
strongly recommended that the input capacitor (C
IN
)be
placed as close as possible to the LM2750, so that the traces
from the input pads are as short and straight as possible. To
minimize the effect of input noise on LM2750 performance, it
is best to bring two traces out from the LM2750 all the way to
the input capacitor pad, so that they are connected at the
capacitor pad. Connecting the two input traces between the
input capacitor and the LM2750 input pads could make the
LM2750 more susceptible to noise-related performance degradation. It is also recommended that the input capacitor be
on the same side of the PCB as the LM2750, and that traces
remain on this side of the board as well (vias to traces on
other PCB layers are not recommended between the input
capacitor and LM2750 input pads).
The two output pads, pads 1 and 2, must also be connected
externally. It is recommended that the output capacitor
(C
OUT
) be placed as close to the LM2750 output pads as
possible. It is best if routing of output pad traces follow
guidelines similar to those presented for the input pads and
capacitor. The flying capacitor (C
FLY
) should also be placed
as close to the LM2750 as possible to minimize PCB trace
length between the capacitor and the IC. Due to the padlayout of the part, it is likely that the trace from one of the
flying capacitor pads (C+ or C-) will need to be routed to an
internal or opposite-side layer using vias. This is acceptable,
and it is much more advantageous to route a flying capacitor
trace in this fashion than it is to place input traces on other
layers.
The following pads of the LM2750 are ground connections
and must be connected externally: pads 3, 5, 6 and the
die-attach pad (DAP). Large, low impedance copper fills and
via connections to an internal ground plane are the preferred
way of connecting together the ground pads of the LM2750,
the input capacitor, and the output capacitor, as well as
connecting this circuit ground to the system ground of the
PCB.
SHUTDOWN
When the voltage on the active-low-logic shutdown pin is
low, the LM2750 will be in shutdown mode. In shutdown, the
LM2750 draws virtually no supply current. There is a 200kΩ
pull-down resistor tied between the SD pin and GND that
pulls the SD pin voltage low if the pin is not driven by a
voltage source. When pulling the part out of shutdown, the
voltage source connected to the SD pin must be able to drive
the current required by the 200kΩ resistor. For voltage management purposes required upon startup, internal switches
connect the output of the LM2750 to an internal pull-down
resistor (1kΩ typ) when the part is shutdown. Driving the
output of the LM2750 by another supply when the LM2750 is
shutdown is not recommended, as the pull-down resistor
was not sized to sink continuous current.
SOFT START
The LM2750 employs soft start circuitry to prevent excessive
input inrush currents during startup. The output voltage is
programmed to rise from 0V to the nominal output voltage
(5.0V) in 500µs (typ.). Soft-start is engaged when a part, with
input voltage established, is taken out of shutdown mode by
pulling the SD pin voltage high. Soft-start will also engage
when voltage is established simultaneously to the input and
SD pins.
OUTPUT CURRENT CAPABILITY
The LM2750 is guaranteed to provide 120mA of output
current when the input voltage is within 2.9V-to-5.6V. Using
the LM2750 to drive loads in excess of 120mA is possible.
IMPORTANT NOTE: Understanding relevant application issues is recommended and a thorough analysis of the application circuit should be performed when using the part outside operating ratings and/or specifications to ensure
satisfactory circuit performance in the application. Special
care should be paid to power dissipation and thermal effects.
These parameters can have a dramatic impact on highcurrent applications, especially when the input voltage is
high. (see "Power Efficiency and Power Dissipation" section,
to come).
The schematic of Figure 1 is a simplified model of the
LM2750 that is useful for evaluating output current capability.
The model shows a linear pre-regulation block (Reg), a
voltage doubler (2x), and an output resistance (R
OUT
). Output resistance models the output voltage droop that is inherent to switched capacitor converters. The output resistance
of the LM2750 is 5Ω (typ.), and is approximately equal to
twice the resistance of the four LM2750 switches. When the
output voltage is in regulation, the regulator in the model
controls the voltage V’ to keep the output voltage equal to
5.0V
±
4%. With increased output current, the voltage drop
across R
OUT
increases. To prevent droop in output voltage,
the voltage drop across the regulator is reduced, V’ increases, and V
OUT
remains at 5V. When the output current
LM2750
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Page 8
Operation Description (Continued)
increases to the point that there is zero voltage drop across
the regulator, V’ equals the input voltage, and the output
voltage is "on the edge" of regulation. Additional output
current causes the output voltage to fall out of regulation,
and the LM2750 operation is similar to a basic open-loop
doubler. As in a voltage doubler, increase in output current
results in output voltage drop proportional to the output
resistance of the doubler. The out-of-regulation LM2750 output voltage can be approximated by:
V
OUT
= 2xVIN-I
OUTxROUT
Again, this equation only applies at low input voltage and
high output current where the LM2750 is not regulating. See
Output Current vs. Output Voltage curves in the Typical
Performance Characteristics section for more details.
A more complete calculation of output resistance takes into
account the effects of switching frequency, flying capacitance, and capacitor equivalent series resistance (ESR).
This equation is shown below:
Switch resistance (5Ω typ.) dominates the output resistance
equation of the LM2750. With a 1.7MHz typical switching
frequency, the 1/(FxC) component of the output resistance
contributes only 0.6Ω to the total output resistance. Increasing the flying capacitance will only provide minimal improvement to the total output current capability of the LM2750. In
some applications it may be desirable to reduce the value of
the flying capacitor below 1µF to reduce solution size and/or
cost, but this should be done with care so that output resistance does not increase to the point that undesired output
voltage droop results. If ceramic capacitors are used, ESR
will be a negligible factor in the total output resistance, as the
ESR of quality ceramic capacitors is typically much less than
100mΩ.
THERMAL SHUTDOWN
The LM2750 implements a thermal shutdown mechanism to
protect the device from damage due to overheating. When
the junction temperature rises to 150
o
C (typ.), the part
switches into shutdown mode. The LM2750 releases thermal
shutdown when the junction temperature of the part is reduced to 130
o
C (typ.).
Thermal shutdown is most-often triggered by self-heating,
which occurs when there is excessive power dissipation in
the device and/or insufficient thermal dissipation. LM2750
power dissipation increases with increased output current
and input voltage (see Power Efficiency and Power Dissi-
pation section). When self-heating brings on thermal shutdown, thermal cycling is the typical result. Thermal cycling is
the repeating process where the part self-heats, enters thermal shutdown (where internal power dissipation is practically
zero), cools, turns-on, and then heats up again to the thermal shutdown threshold. Thermal cycling is recognized by a
pulsing output voltage and can be stopped be reducing the
internal power dissipation (reduce input voltage and/or output current) or the ambient temperature. If thermal cycling
occurs under desired operating conditions, thermal dissipation performance must be improved to accommodate the
power dissipation of the LM2750. Fortunately, the LLP package has excellent thermal properties that, when soldered to
a PCB designed to aid thermal dissipation, allows the
LM2750 to operate under very demanding power dissipation
conditions.
OUTPUT CURRENT LIMITING
The LM2750 contains current limit circuitry that protects the
device in the event of excessive output current and/or output
shorts to ground. Current is limited to 300mA (typ.) when the
output is shorted directly to ground. When the LM2750 is
current limiting, power dissipation in the device is likely to be
quite high. In this event, thermal cycling should be expected
(see Thermal Shutdown section).
Application Information
OUTPUT VOLTAGE RIPPLE
The amount of voltage ripple on the output of the LM2750 is
highly dependent on the application conditions: output current and the output capacitor, specifically. A simple approximation of output ripple is determined by calculating the
amount of voltage droop that occurs when the output of the
LM2750 is not being driven. This occurs during the charge
phase (φ1). During this time, the load is driven solely by the
charge on the output capacitor. The magnitude of the ripple
thus follows the basic discharge equation for a capacitor (I =
C x dV/dt), where discharge time is one-half the switching
period, or 0.5/F
SW
. Put simply,
A more thorough and accurate examination of factors that
affect ripple requires including effects of phase non-overlap
times and output capacitor equivalent series resistance
(ESR). In order for the LM2750 to operate properly, the two
phases of operation must never coincide. (If this were to
happen all switches would be closed simultaneously, shorting input, output, and ground). Thus, non-overlap time is built
into the clocks that control the phases. Since the output is
not being driven during the non-overlap time, this time
should be accounted for in calculating ripple. Actual output
capacitor discharge time is approximately 60% of a switching period, or 0.6/F
SW
.
The ESR of the output capacitor also contributes to the
output voltage ripple, as there is effectively an AC voltage
drop across the ESR due to current switching in and out of
the capacitor. The following equation is a more complete
calculation of output ripple than presented previously, taking
into account phase non-overlap time and capacitor ESR.
20035109
FIGURE 1. LM2750 Output Resistance Model
LM2750
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Application Information (Continued)
A low-ESR ceramic capacitor is recommended on the output
to keep output voltage ripple low. Placing multiple capacitors
in parallel can reduce ripple significantly, both by increasing
capacitance and reducing ESR. When capacitors are in
parallel, ESR is in parallel as well. The effective net ESR is
determined according to the properties of parallel resistance.
Two identical capacitors in parallel have twice the capacitance and half the ESR as compared to a single capacitor of
the same make. On a similar note, if a large-value, high-ESR
capacitor (tantalum, for example) is to be used as the primary output capacitor, the net output ESR can be significantly reduced by placing a low-ESR ceramic capacitor in
parallel with this primary output capacitor.
CAPACITORS
The LM2750 requires 3 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (≤10mΩ typ.).
Tantalum capacitors, OS-CON capacitors, and aluminum
electrolytic capacitors generally are not recommended for
use with the LM2750 due to their high ESR, as compared to
ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2750. These capacitors have tight capacitance tolerance
(as good as
±
10%), hold their value over temperature (X7R:
±
15% over -55oCto125oC; X5R:±15% over -55oCto85oC),
and typically have little voltage coefficient. Capacitors with
Y5V and/or Z5U temperature characteristic are generally not
recommended. These types of capacitors typically have
wide capacitance tolerance (+80%, -20%), vary significantly
over temperature (Y5V: +22%, -82% over -30
o
C to +85oC
range; Z5U: +22%, -56% over +10
o
Cto+85oC range), and
have poor voltage coefficients. Under some conditions, a
nominal 1µF Y5V or Z5U capacitor could have a capacitance
of only 0.1µF. Such detrimental deviation is likely to cause
these Y5V and Z5U of capacitors to fail to meet the minimum
capacitance requirements of the LM2750.
The table below lists some leading ceramic capacitor manufacturers.
Manufacturer Contact Information
TDK www.component.tdk.com
AVX www.avx.com
Murata www.murata.com
Taiyo-Yuden www.t-yuden.com
Vishay-Vitramon www.vishay.com
INPUT CAPACITORS
The input capacitor (C
IN
) is used as a reservoir of charge,
helping to quickly transfer charge to the flying capacitor
during the charge phase (φ1) of operation. The input capacitor helps to keep the input voltage from drooping at the start
of the charge phase, when the flying capacitor is first connected to the input, and helps to filter noise on the input pin
that could adversely affect sensitive internal analog circuitry
biased off the input line. As mentioned above, an X7R/X5R
ceramic capacitor is recommended for use. For applications
where the maximum load current required is between 60mA
and 120mA, a minimum input capacitance of 2.0µF is required. For applications where the maximum load current is
60mA or less, 1.0µF of input capacitance is sufficient. Failure
to provide enough capacitance on the LM2750 input can
result in poor part performance, often consisting of output
voltage droop, excessive output voltage ripple and/or excessive input voltage ripple.
FLYING CAPACITOR
The flying capacitor (C
FLY
) transfers charge from the input to
the output, providing the voltage boost of the doubler. A
polarized capacitor (tantalum, aluminum electrolytic, etc.)
must not be used here, as the capacitor will be reversebiased upon start-up of the LM2750. The size of the flying
capacitor and its ESR affect output current capability when
the input voltage of the LM2750 is low, most notable for input
voltages below 3.0V. These issues were discussed previously in the Output Current Capability section. For most
applications, a 1µF X7R/X5R ceramic capacitor is recommended for the flying capacitor.
OUTPUT CAPACITOR
The output capacitor of the LM2750 plays an important part
in determining the characteristics of the output signal of the
LM2750, many of which have already been discussed. The
ESR of the output capacitor affects charge pump output
resistance, which plays a role in determining output current
capability. Both output capacitance and ESR affect output
voltage ripple. For these reasons, a low-ESR X7R/X5R ceramic capacitor is the capacitor of choice for the LM2750
output.
In addition to these issues previously discussed, the output
capacitor of the LM2750 also affects control-loop stability of
the part. Instability typically results in the switching frequency effectively reducing by a factor of two, giving excessive output voltage droop and/or increased voltage ripple on
the output and the input. With output currents of 60mA or
less, a minimum capacitance of 1.0µF is required at the
output to ensure stability. For output currents between 60mA
and 120mA, a minimum output capacitance of 2.0µF is
required.
POWER EFFICIENCY AND POWER DISSIPATION
Efficiency of the LM2750 mirrors that of an unregulated
switched capacitor converter followed by a linear regulator.
The simplified power model of the LM2750, in Figure 2, will
be used to discuss power efficiency and power dissipation.
In calculating power efficiency, output power (P
OUT
) is easily
determined as the product of the output current and the 5.0V
output voltage. Like output current, input voltage is an
application-dependent variable. The input current can be
calculated using the principles of linear regulation and
switched capacitor conversion. In an ideal linear regulator,
the current into the circuit is equal to the current out of the
circuit. The principles of power conservation mandate the
ideal input current of a voltage doubler must be twice the
output current. Adding a correction factor for operating quiescent current (I
Q
, 5mA typ.) gives an approximation for total
input current which, when combined with the other input and
output parameter(s), yields the following equation for efficiency:
Comparisons of LM2750 efficiency measurements to calculations using the above equation have shown the equation to
be a quite accurate approximation of actual efficiency. Be-
LM2750
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Page 10
Application Information (Continued)
cause efficiency is inversely proportional to input voltage, it
is highest when the input voltage is low. In fact, for an input
voltage of 2.9V, efficiency of the LM2750 is greater than 80%
(I
OUT
≥ 40mA) and peak efficiency is 85% (I
OUT
= 120mA).
The average efficiency for an input voltage range spanning
the Li-Ion range (2.9V-to-4.2V) is 70% (I
OUT
= 120mA). At
higher input voltages, efficiency drops dramatically. In Li-Ionpowered applications, this is typically not a major concern,
as the circuit will be powered off a charger in these circumstances. Low efficiency equates to high power dissipation,
however, which could become an issue worthy of attention.
LM2750 power dissipation (P
D
) is calculated simply by sub-
tracting output power from input power:
P
D=PIN-POUT
=[VINx (2·I
OUT+IQ
)]-[V
OUTxIOUT
]
Power dissipation increases with increased input voltage
and output current, up to 772mW at the ends of the operating
ratings (V
IN
= 5.6V, I
OUT
= 120mA). Internal power dissipation self-heats the device. Dissipating this amount power/
heat so the LM2750 does not overheat is a demanding
thermal requirement for a small surface-mount package.
When soldered to a PCB with layout conducive to power
dissipation, the excellent thermal properties of the LLP package enable this power to be dissipated from the LM2750 with
little or no derating, even when the circuit is placed in elevated ambient temperatures (see Optimizing Layout for
Thermal Dissipation section).
20035110
FIGURE 2. LM2750 Model for Power Efficiency and
Power Dissipation Calculations
LM2750
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Page 11
Physical Dimensions inches (millimeters) unless otherwise noted
10-Pin LLP
NS Package Number LDA10A
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LM2750 Low Noise, 5.0V Regulated Switched Capacitor Voltage Converter
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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