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LM3350MMX Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM3350MMX
Description  Switched Capacitor Voltage Converter
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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Detailed Operation (Continued)
STEP-UP APPLICATIONS NEED AN EXTRA DIODE
The LM3350 is biased from pin V
high. Thus for step-up appli-
cations, an external Schottky diode (D1) is needed to supply
power to V
high during startup (See Figure 2). Note that during
shutdown, this diode will provide a DC path from V
in to Vout.
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
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 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 nor-
mal operation.
Application Information
CAPACITOR SELECTION
The LM3350 requires four capacitors: an input bypass ca-
pacitor (connected between V
in and ground), an output hold
capacitor (connected between V
out 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 (C
in) 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 (Z
out)
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 (C
out) can be calculated
(approximately) based on the output ripple (
∆V
out) require-
ments from:
where I
load is the load current and fO is the oscillator fre-
quency.
In order to ensure superior performance over the entire oper-
ating 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.
DS100002-11
A. Step-Up Mode
DS100002-12
B. Step-Down Mode
FIGURE 1. Z
out vs. C
www.national.com
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