Page 1
October 2002
LM2788
120mA High Efficiency Step-Down Switched Capacitor
Voltage Converter
LM2788 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter
General Description
The LM2788 switched capacitor step-down DC/DC converter efficiently produces a 120mA regulated low-voltage
rail from a 2.6V to 5.5V input. Fixed output voltage options of
1.5V, 1.8V, and 2.0V are available. The LM2788 uses multiple fractional gain configurations to maximize conversion
efficiency over the entire input voltage and output current
ranges. Also contributing to high overall efficiency is the
extremely low supply current of the LM2788: 32µA operating
unloaded and 0.1µA in shutdown.
The optimal external component requirements of the
LM2788 solution minimize size and cost, making the part
ideal for Li-Ion and other battery powered designs. Two 1µF
flying capacitors and two 10µF bypass capacitors are all that
are required, and no inductors are needed.
The LM2788 also features noise-reducing soft-start circuitry,
short-circuit protection and over-temperature protection.
Typical Application Circuit
Features
n Output voltage options:
n 120mA output current capability
n Multi-Gain and Gain Hopping for Highest Possible
n 2.6V to 5.5V input range
n Low operating supply current: 32µA
n Shutdown supply current: 0.1µA
n Thermal and short circuit protection
n Available in an 8-Pin MSOP Package
±
5%, 1.8V±5%, 1.5V±6%
2.0V
Efficiency - up to 90% Efficient
Applications
n Cellular Phones
n Pagers
n H/PC and P/PC Devices
n Portable Electronic Equipment
n Handheld Instrumentation
20044401
© 2002 National Semiconductor Corporation DS200444 www.national.com
Page 2
Connection Diagram
LM2788
LM2788
Mini SO-8 (MSOP-8) Package
NS Package #: MUA08A
Pin Description
Pin Name Description
1V
2 C1- First Flying Capacitor: Negative Terminal
3 C1+ First Flying Capicitor: Positive terminal
4V
5 EN Enable. Logic Input. High voltage = ON, Low voltage = SHUTDOWN
6 C2+ Second Flying-Capacitor: Positive Terminal
7 GND Ground Connection
8 C2- Second Flying Capacitor: Negative Terminal
OUT
IN
Ordering Information
Output
Voltage
1.50V LM2788MM-1.5
1.80V LM2788MM-1.8 S23B 1000 units on Tape-and Reel
2.00V LM2788MM-2.0 S24B 1000 units on Tape-and Reel
Ordering
Information
LM2788MMX-1.5 S30B 3500 units onTape-and-Reel
LM2788MMX-1.8 S23B 3500 units on Tape-and Reel
LM2788MMX-2.0 S24B 3500 units on Tape-and Reel
Top View
20044402
Regulated Output Voltage
Input voltage. Recommended VINRange: 2.6V to 5.5V
Package Type Package Marking Supplied as
S30B 1000 units on Tape-and Reel
MSOP-8
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Page 3
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.
, EN pins: Voltage to Ground
V
IN
(Note 3) −0.3V to 5.6V
Junction Temperature (T
J-MAX-ABS
Continuous Power Dissipation
(Note 4) Internally Limited
V
Short-Circuit to GND Duration
OUT
(Note 4) Unlimited
Storage Temperature Range −65˚C to 150˚C
Lead Temperature
(Soldering, 5 Sec.) 260˚C
ESD Rating (Note 5)
Human-body model:
Machine model
) 150˚C
2kV
200V
Operating Ratings (Notes 1, 2)
Input Voltage Range 2.6V to 5.5V
Recommended Output Current
Range 0mA to 120mA
Junction Temperature Range -40˚C to 125˚C
Ambient Temperature Range
-40˚C to 85˚C
(Note 6)
Thermal Information
Junction-to-Ambient Thermal 220˚C/W
Resistance, MSOP-8 Package
(θJA) (Note 7)
LM2788
Electrical Characteristics (Notes 2, 8) Limits in standard typeface and typical values apply for T
o
C. Limits in boldface type apply over the operating junction temperature range. Unless otherwise specified: 2.6 ≤ VIN≤
25
5.5V, V(EN) = V
IN,C1=C2
= 1µF, CIN=C
= 10µF. (Note 9)
OUT
=
J
Symbol Parameter Conditions Min Typ Max Units
LM2788-1.8, LM2788-2.0
2.8V ≤ V
V
OUT
Output Voltage Tolerance
0mA ≤ I
4.2V ≤ VIN≤ 5.5V
0mA ≤ I
IN
OUT
OUT
≤ 4.2V
≤ 120mA
≤ 120 mA
-5 +5
-6 +6
%of
V
OUT (nom)
(Note 10)
LM2788-1.5
2.8V ≤ V
V
OUT
Output Voltage Tolerance
0mA ≤ I
4.2V ≤ VIN≤ 5.5V
0mA ≤ I
IN
OUT
OUT
≤ 4.2V
≤ 120 mA
≤ 120mA
-6 +6
-6 +6
%of
V
OUT (nom)
(Note 10)
All Output Voltage Options
I
I
V
E
E
t
f
I
Q
SD
R
PEAK
AVG
ON
SW
SC
Operating Supply Current I
= 0mA 32 50 µA
OUT
Shutdown Supply Current V(EN) = 0V 0.1 2 µA
Output Voltage Ripple LM2788-1.8: VIN= 3.6V, I
Peak Efficiency LM2788-1.8: VIN= 3.0V, I
Average Efficiency over
Li-Ion Input Voltage Range
(Note 11)
Turn-On Time VIN= 3.6V, I
LM2788-1.5: 3.0 ≤ V
LM2788-2.0: 3.0 ≤ V
= 120mA (Note 12) 0.4 ms
OUT
IN
IN
IN
= 120mA 20 mV
OUT
= 60mA 90 %
OUT
≤ 4.2V, I
≤ 4.2V, I
≤ 4.2V, I
= 60mA 76
OUT
= 60mA 82
OUT
= 60mA 75
OUT
Switching Frequency 500 kHz
Short-Circuit Current VIN= 3.6, V
=0V 25 mA
OUT
Enable Pin (EN) Characteristics
V
IH
V
IL
I
EN
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: Voltage on the EN pin must not be brought above V
EN pin Logic-High Input 0.9 V
IN
EN pin Logic-Low Input 0 0.4 V
=0V 0 nA
V
EN pin input current
EN
V
= 5.5V 30
EN
+ 0.3V.
IN
p-p
%LM2788-1.8: 3.0 ≤ V
V
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Page 4
Electrical Characteristics (Notes 2, 8) Limits in standard typeface and typical values apply for T
Limits in boldface type apply over the operating junction temperature range. Unless otherwise specified: 2.6 ≤ V
LM2788
V(EN) = V
Note 4: Thermal shutdown circuitry protects the device from permanent damage.
Note 5: 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 6: Maximum ambient temperature (T
dissipation of the device in the application (P
following equation: T
outside the listed T
Note 7: Junction-to-ambient thermal resistance is a highly application and board-layout dependent. In applications where high maximum power dissipation exists,
special care must be paid to thermal dissipation issues. Fore more information on these topics, please refer to the Power Dissipation section of this datasheet.
Note 8: 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 9: C
Note 10: Nominal output voltage (V
table for available options.
Note 11: Efficiency is measured versus V
results. Weighting to account for battery voltage discharge characteristics (V
Note 12: Turn-on time is measured from when the EN signal is pulled high until the output voltage crosses 90% of its final value.
IN,C1=C2
FLY,CIN
= 1µF, CIN=C
A-MAX=TJ-MAX-OP
rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 125oC.
A
, and C
OUT
-(θJAxP
: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
(nom) ) is the target output voltage of the part, as given by the output-voltage-option identifier. See Ordering Information
OUT
= 10µF. (Note 9) (Continued)
OUT
) is dependent on the maximum operating junction temperature (T
A-MAX
), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the
D-MAX
). The ambient temperature operating rating is provided merely for convenience. This part may be operated
D-MAX
, with VINbeing swept in small increments from 3.0V to 4.2V. The average is calculated from these measurements
IN
vs. Time) is not done in computing the average.
BAT
J-MAX-OP
= 125oC), the maximum power
Block Diagram
IN
=25oC.
J
≤ 5.5V,
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20044403
Page 5
Typical Performance Characteristics
Unless otherwise specified: CIN= 10µF, C1 = 1.0µF, C2 = 1.0µF C
ceramic capacitors (MLCC’s).
Output Voltage vs. Input Voltage:
LM2788-1.5 (1mA)
20044407 20044408
= 10µF, TA=25oC. Capacitors are low-ESR multi-layer
OUT
Output Voltage vs. Input Voltage:
LM2788-1.5 (120mA)
LM2788
Output Voltage vs. Input Voltage:
LM2788-1.8 (1mA)
Output Voltage vs. Input Voltage:
LM2788-2.0 (1mA)
Output Voltage vs. Input Voltage:
LM2788-1.8 (120mA)
20044409 20044410
Output Voltage vs. Input Voltage:
LM2788-2.0 (120mA)
20044411 20044412
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Page 6
Typical Performance Characteristics (Continued)
LM2788
Efficiency vs. Input Voltage: LM2788-1.5 Efficiency vs. Output Current: LM2788-1.5
20044413
20044414
Efficiency vs. Input Voltage: Lm2788-1.8 Efficiency vs. Output Current: LM2788-1.8
20044415
20044416
Efficiency vs. Input Voltage: LM2788-2.0 Effiency vs. Output Current: LM2788-2.0
20044417
20044418
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Page 7
Typical Performance Characteristics (Continued)
Output Voltage Ripple vs. Output Current Output Voltage Ripple vs. Input Voltage
20044421 20044419
Output Voltage Ripple Short Circuit Current
LM2788
20044406
Start Up Waveform Transient Load Response
20044404 20044405
20044420
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Page 8
Operation Description
LM2788
OVERVIEW
The LM2788 is a switched capacitor converter that produces
a regulated low-voltage output. The core of the part is the
highly efficient charge pump that utilizes multiple fractional
gains and pulse-frequency modulated (PFM) switching to
minimize power losses over wide input voltage and output
current ranges. A description of the principal operational
characteristics of the LM2788 is broken up into the following
sections: PFM Regulation, Fractional Multi-Gain Charge
Pump, and Gain Selection for Optimal Efficiency. Each of
these sections refers to the block diagram presented on the
previous page.
PFM REGULATION
The LM2788 achieves tightly regulated output voltages with
pulse-frequency modulated (PFM) regulation. PFM simply
means the part only pumps when it needs to. When the
output voltage is above the target regulation voltage, the part
idles and consumes minimal supply-current. In this state, the
load current is supplied solely by the charge stored on the
output capacitor. As this capacitor discharges and the output
voltage falls below the target regulation voltage, the charge
pump activates. Charge/current is delivered to the output
(supplying the load and boosting the voltage on the output
capacitor).
The primary benefit of PFM regulation is when output currents are light and the part is predominantly in the lowsupply-current idle state. Net supply current is minimal because the part only occasionally needs to refresh the output
capacitor by activating the charge pump, and the supply
current it consumes.
FRACTIONAL MULTI-GAIN CHARGE PUMP
The core of the LM2788 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The
charge pump operates by using the external flying capacitors, C1 and C2, to transfer charge from the input to the
output. During the charge phase, which doubles as the PFM
’idle state’, the flying capacitors are charged by the input
supply. The charge pump will be in this state until the output
voltage drops below the target regulation voltage, triggering
the charge pump to activate so that it can deliver charge to
the output. Charge transfer is achieved in the pump phase,
where the fully charged flying capacitors are connected to
the output so that the charge they hold can supply the load
and recharge the output capacitor.
Input, output, and intermediary connections of the flying
capacitors are made with internal MOS switches. The
LM2788 utilizes two flying capacitors and a versatile switch
network to achieve several fractional voltage gains:
and 1. With this gain-switching ability, it is as if the LM2788
is three-charge-pumps-in-one. The ’active’ charge pump at
any given time is the one that will yield the highest efficiency
given the input and output conditions present.
1
⁄2,2⁄3,
E=(V
OUTxIOUT
)÷(VINxIIN)=V
÷(GXVIN)
OUT
In the equations, G represents the charge pump gain. Efficiency is optimal as GxV
approaches V
IN
. Optimal effi-
OUT
ciency is achieved when gain is able to adjust depending on
input and output voltage conditions. Due to the nature of
charge pumps, G cannot adjust continuously, which would
be ideal from an efficiency standpoint. But G can be a set of
simple quantized ratios, allowing for a good degree of efficiency optimization.
2
The gain set of the LM2788 consists of the gains 1/2,
⁄3, and
1. An internal input voltage range detector, along with the
nominal output voltage of the given LM2788 option, determines what is to be referred to as the ’base gain’ of the part,
. The base gain is the default gain configuration of the part
G
B
at a given V
. Table 1 lists GBof the LM2788-1.8 over the
IN
input voltage range. (For the remainder of this discussion,
the 1.8V option of the LM2788 will be used as an example.
The other voltage options operate under the same principles
as the 1.8V version, the gain-transitions merely occur at
different voltage levels.)
TABLE 1. LM2788-1.8 Base Gain (G
) vs. V
B
IN
Input Voltage Base Gain (GB)
2.6V - 2.9V 1
2.9V - 3.8V
3.8V - 5.5V
2
⁄
3
1
⁄
2
Table 1 shows the efficiency of the LM2788-1.8 versus input
voltage, with output currents of 10mA and 120mA. The base
gain regions (G
set of ideal efficiency gradients, E
) are separated and labeled. There is also a
B
IDEAL(G=xx)
, showing the
ideal efficiency of a charge pumps with gains of 1/2, 2/3, and
1. These curves were generated using the ideal efficiency
equation presented above.
20044422
GAIN SELECTION AND GAIN HOPPING FOR OPTIMAL EFFICIENCY
The ability to switch gains based on input and output conditions results in optimal LM2788 efficiency throughout the
operating ranges of the part. Charge-pump efficiency is derived in the following two ideal equations (supply current and
other losses are neglected for simplicity):
=GxI
I
IN
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OUT
FIGURE 1. Efficiency of LM2788-1.8 with 10mA and
120mA output currents Base-gain (G
) regions are
B
separated and labeled Ideal efficiency curves of
charge pumps with G =1/2, 2/3, and 1 are included
(E
IDEAL(G=1),EIDEAL(G=2/3),EIDEAL(G=1/2)
)
The 10mA-load efficiency curve in Figure 1 closely resembles the ideal Efficiency-vs.-Input- Voltage curves that
correspond to each of the base-gain regions. The same
Page 9
Operation Description (Continued)
holds true for the other base-gain regions. At the base-gain
transitions (V
transition as the part switches base-gains. The 10mA load
curve gives a clear picture of how base-gain affects overall
converter efficiency. With a 10mA output current, the gain of
the LM2788-1.8 is equal to the base-gain over the entire
operating input voltage range. Additionally, with a 10mA load,
internal supply current has a minimal impact on efficiency
(Supply current does have a small affect: it is why the 10mA
load curve is slightly below the ideal efficiency gradients in
each of the base-gain regions).
The 120mA-load curve in Figure 1 illustrates the effect of
gain hopping on converter efficiency. Gain hopping is implemented to overcome output voltage droop that results from
charge-pump non-idealities. In an ideal charge pump, the
output voltage is equal to the product of the gain and the
input voltage. Non-idealities such as finite switch resistance,
capacitor ESR, and other factors result in the output of
practical charge pumps being below the ideal value, however. This output droop is typically modeled as an output
resistance, R
creases linearly with load current.
Real Charge Pump: V
The LM2788 compensates for output voltage droop under
high load conditions by gain hopping: when the base-gain is
not sufficient to keep the output voltage in regulation, the
part will temporarily switch up to the next highest gain setting
to provide an intermittent boost in output voltage. When the
output voltage is sufficiently boosted, the gain configuration
reverts back to the base-gain setting. If the load remains
high, the part will continue to hop back and forth between the
base-gain and the next highest gain setting, and the output
voltage will remain in regulation. In contrast to the base-gain
decision, which is made based on the input voltage, the
decision to gain hop is made by monitoring the voltage at the
output of the part.
The efficiency curve of the LM2788-1.8 with a 120mA output
current, also contained in Figure 1, shows the effect that gain
hopping has on efficiency. Comparing the 120mA load curve
to the 10mA load curve, it is plain to see that to the right of
the base-gain transitions, the efficiency of the 120mA curve
increases gradually whereas the 10mA curve makes a sharp
transition. The base-gain of both curves is the same for both
loads. The difference comes in gain hopping. With the
120mA load, the part will spend a percentage of time in the
base-gain setting and the rest of the time in the next-highest
gain setting. The percentage of time gain hopping decreases
as the input voltage rises, as less gain-hopping boost is
required with increased input voltage. When the input voltage in a given base-gain region is large enough so that no
extra boost from gain hopping is required, the 120mA-load
efficiency curve mirrors the 10mA efficiency curve.
TABLE 2. LM2788-1.8 Gain Hopping Regions
Input Voltage Base Gain
Gain hopping contributes to the overall high efficiency of the
LM2788. Gain hopping only occurs when required for keeping the output voltage in regulation. This allows the LM2788
= 2.9V, 3.8V), the 10mA curve makes sharps
IN
, because the magnitude of the droop in-
OUT
Ideal Charge Pump: V
=(GxVIN)-(I
OUT
OUT
=GxV
Gain Hop
)
(G
B
3.0V - 3.3V
3.8V - 4.4V
2
⁄
3
1
⁄
2
IN
OUTxROUT
Setting
1
2
⁄
3
)
to operate in the higher efficiency base-gain setting as much
as possible. Gain hopping also allows the base-gain transitions to be placed at input voltages that are as low as
practically possible. This maximizes the peaks, and minimizes the valleys, of the efficiency ’saw-tooth’ curves, again
maximizing total solution efficiency.
SHUTDOWN
The LM2788 is in shutdown mode when the voltage on the
active-low logic enable pin (EN) is low. In shutdown, the
LM2788 draws virtually no supply current. When in shutdown, the output of the LM2788 is completely disconnected
from the input, and will be 0V unless driven by an outside
source.
In some applications, it may be desired to disable the
LM2788 and drive the output pin with another voltage
source. This can be done, but the voltage on the output pin
of the LM2788 must not be brought above the input voltage.
The output pin will draw a small amount when driven externally due the internal feedback resistor divider connected
between V
OUT
and GND.
SOFT START
The LM2788 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 in
approximately 400µs (typ.). With the input voltage established, soft-start is engaged when a part is enabled by
pulling the voltage on the EN pin high. Soft-start also engages when voltage is established simultaneously to the V
and EN pins
THERMAL SHUTDOWN
Protection from overheating-related damage is achieved
with a thermal shutdown feature. When the junction temperature rises to 150
o
C (typ.), the part switches into shutdown mode. The LM2788 disengages thermal shutdown
when the junction temperature of the part is reduced to
o
C (typ.). Due to its high efficiency, the LM2788 should
130
not activate thermal shutdown (or exhibit related thermal
cycling) when the part is operated within specified input
voltage, output current, and ambient temperature operating
ratings.
SHORT-CIRCUIT PROTECTION
The LM2788 short-circuit protection circuitry that protects the
device in the event of excessive output current and/or output
shorts to ground. A graph of ’Short-Circuit Current vs. Input
Voltage’ is provided in the Performance Characteristics
section.
Application Information
OUTPUT VOLTAGE RIPPLE
The voltage ripple on the output of the LM2788 is highly
dependent on the application conditions. The output capacitor, the input voltage, and the output current each play a
significant part in determining the output voltage ripple. Due
to the complexity of LM2788 operation, providing equations
or models to approximate the magnitude of the ripple cannot
be easily accomplished. The following general statements
can be made however
The output capacitor will have a significant effect on output
voltage ripple magnitude. Ripple magnitude will typically be
linearly proportional to the output capacitance present. A
LM2788
IN
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Page 10
Application Information (Continued)
low-ESR ceramic capacitor is recommended on the output to
LM2788
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.
Ripple is increased when the LM2788 is gain hopping. Thus,
in the presence of high currents, ripple is likely to vary
significantly over the input voltage, depending on wether or
not the part is gain hopping.
CAPACITORS
The LM2788 requires 4 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 (ESR, ≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended
for use with the LM2788 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
LM2788. These capacitors have tight capacitance tolerance
(as good as
±
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 for use with the LM2788.
These types of capacitors typically have wide capacitance
tolerance (+80%, -20%), vary significantly over temperature
(Y5V: +22%, -82% over -30
-56% over +10
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 LM2788.
The table below lists some leading ceramic capacitor manufacturers.
Vishay-Vitramon www.vishay.com
OUTPUT CAPACITOR
The output capacitor of the LM2788 plays an important part
in LM2788 performance. In typical high-current applications,
a 10µF low-ESR (ESR = equivalent series resistance) ceramic capacitor is recommended for use. For lighter loads,
the output capacitance may be reduced (capacitance as low
as 1µF for output currents ≤ 60mA is usually acceptable).
The performance of the part should be evaluated with spe-
±
10%), hold their value over temperature (X7R:
o
o
C to +85oC range), and have poor voltage
Cto+85oC range; Z5U: +22%,
Manufacturer Contact Information
AVX www.avx.com
Murata www.murata.com
Taiyo-Yuden www.t-yuden.com
TDK www.component.tdk.com
cial attention paid to efficiency and output ripple to ensure
the capacitance chosen on the output yields performance
suitable for the application. In extreme cases, excessive
ripple could cause control loop instability, severely affecting
the performance of the part. If excessive ripple is present,
the output capacitance should be increased.
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 (See Output Voltage Ripple section, above).
For these reasons, a low-ESR X7R/X5R ceramic capacitor is
the capacitor of choice for the LM2788 output.
FLYING CAPACITORS
The flying capacitors (C
and C2) transfer charge from the
1
input to the output, and thus are like the engine of the charge
pump. Low-ESR ceramic capacitors with X7R or X5R temperature characteristic are strongly recommended for use
here. The flying capacitors C1 and C2 should be identical. As
a general rule, the capacitance value of each flying capacitor
should be 1/10th that of the output capacitor. Polarized
capacitor (tantalum, aluminum electrolytic, etc.) must not be
used for the flying capacitors, as they could become reversebiased upon start-up of the LM2788.
The flying capacitance determines the strength of the charge
pump-the larger the capacitance, the bigger the engine. ESR
in the flying capacitors negatively affects the strength of the
charge pump and should be minimized, as ESR contributes
to undesired output resistance. If capacitors are too small
the LM2788 could spend excessive amount of time gain
hopping: decreasing efficiency, increasing output voltage
ripple, and possibly impeding the ability of the part to regulate. On the other hand, if the flying capacitors are too large
they could potentially overwhelm the output capacitor, resulting in increased output voltage ripple.
INPUT CAPACITOR
If the flying capacitors are the charge pump engine, the input
capacitor (CIN) is the fuel tank: a reservoir of charge that
aids a quick transfer of charge from the supply to the flying
capacitors during the charge phase 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. As a
general recommendation, the input capacitor should be chosen to match the output capacitor.
POWER DISSIPATION
LM2788 power dissipation will, typically, not be much of a
concern in most applications. Derating to accommodate selfheating will rarely be required due to the high efficiency of
the part. When operating within specified operating ratings,
the peak power dissipation (PD) of all LM2788 voltage options occurs with the LM2788-1.5 operating at the maximum
rated operating output current of 120mA. With an input voltage of 5.5V, the power efficiency (E) of the LM2788-1.5
bottoms out at 54%. Assuming a typical junction-to-ambient
thermal resistance (θJA) for the MSOP package of 220˚C/
Watt, the junction temperature (T
) of the part is calculated
J
below for a part operating at the maximum rated ambient
temperature (T
) of 85˚C.
A
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Application Information (Continued)
Even under these peak power dissipation and ambient temperature conditions, the junction temperature of the LM2788
is below the maximum operating rating of 125˚C.
As an additional note, the ambient temperature operating
rating range listed in the specifications is provided merely for
convenience. The LM2788 may be operated outside this
rating, so long as the junction temperature of the device
does not exceed the maximum operating rating of 125˚C.
Use short, wide traces to connect the external capacitors
•
to the LM2788 to minimize trace resistance and inductance.
Use a low resistance connection between ground and the
•
GND pin of the LM2788. Using wide traces and/or multiple vias to connect GND to a ground plane on the board
is most advantageous.
Figure 2 is a sample single-layer board layout that accommodates the LM2788 typical application circuit, as pictured
on the cover of this datasheet
20044424
LM2788
Layout Guidelines
Proper board layout to accommodate the LM2788 circuit will
help to ensure optimal performance. The following guidelines are recommended:
Place capacitors as close to the LM2788 as possible, and
•
preferably on the same side of the board as the IC.
FIGURE 2. Sample single-layer board layout of the
LM2788 Typical Application Circuit (Vias to a ground
plane, assumed to be present, are located in the
center of the LM2788 footprint.)
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Page 12
Physical Dimensions inches (millimeters)
unless otherwise noted
Mini SO-8 (MSOP-8)
MUA08A
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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
LM2788 120mA High Efficiency Step-Down Switched Capacitor Voltage Converter
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
labeling, can be reasonably expected to result in a
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