LM3350 MWC/NOPB National Semiconductor, LM3350 MWC/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM3350 MWC/NOPB
• Manufacturer: National Semiconductor

Specifications of LM3350 MWC/NOPB

• Lead Free Status / Rohs Status: Compliant

Detailed Operation
OPERATING PRINCIPLE
The LM3350 is a charge-pump voltage converter that pro-
vides 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 volt-
ages 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 ex-
plained in the Capacitor Selection section under Application
Information.
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3350 is biased from pin V
cations, an external Schottky diode (D1) is needed to supply
power to V
shutdown, this diode will provide a DC path from V
The load may therefore continue to draw current from the in-
put voltage source. This Schottky diode is not required for
step-down applications (See
SHUTDOWN MODE
When ENABLE is a logic low (ground), the LM3350 enters a
low power shutdown mode. In this mode, all circuitry is dis-
abled and therefore, all switching action stops. During shut-
down, 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 op-
eration.
Application Information
CAPACITOR SELECTION
The LM3350 requires four capacitors: an input bypass ca-
pacitor (connected between V
capacitor (connected between V
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 (C
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.
1
ue.
The output hold capacitor value determines the output ripple.
Increasing the value of the hold capacitor decreases the rip-
shows typical output impedance (Z
high
during startup (See
Figure
in
in
) is the least critical. Its value
and ground), an output hold
high
out
Figure
100002 Version 6 Revision 2
3).
. Thus for step-up appli-
out
and ground), and two
) versus capacitor val-
2). Note that during
in
to V
Figure
out
.
Print Date/Time: 2010/12/28 23:06:13
7
ple. The value of this capacitor (C
proximately) based on the output ripple (ΔV
from:
where I
cy.
In order to ensure superior performance over the entire op-
erating 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.
PRECAUTIONS
The LM3350 is not short circuit protected.
load
is the load current and f
FIGURE 1. Z
B. Step-Down Mode
A. Step-Up Mode
out
out
O
vs. C
) can be calculated (ap-
is the oscillator frequen-
out
10000211
10000212
) requirements
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