LM2750LD-5.0 National Semiconductor, LM2750LD-5.0 Datasheet - Page 8

• Manufacturer Part Number: LM2750LD-5.0
• Description: Low Noise/ 5.0V Regulated Switched Capacitor Voltage Converter
• Manufacturer: National Semiconductor

www.national.com
Operation Description
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 out-
put voltage can be approximated by:
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 capaci-
tance, 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. Increas-
ing the flying capacitance will only provide minimal improve-
ment 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 resis-
tance 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
switches into shutdown mode. The LM2750 releases thermal
shutdown when the junction temperature of the part is re-
duced to 130
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 shut-
down, thermal cycling is the typical result. Thermal cycling is
FIGURE 1. LM2750 Output Resistance Model
o
C (typ.).
V
OUT
= 2xV
IN
- I
OUT
x R
(Continued)
OUT
o
C (typ.), the part
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8
the repeating process where the part self-heats, enters ther-
mal shutdown (where internal power dissipation is practically
zero), cools, turns-on, and then heats up again to the ther-
mal 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 out-
put current) or the ambient temperature. If thermal cycling
occurs under desired operating conditions, thermal dissipa-
tion performance must be improved to accommodate the
power dissipation of the LM2750. Fortunately, the LLP pack-
age 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 cur-
rent and the output capacitor, specifically. A simple approxi-
mation 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
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, short-
ing 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 switch-
ing period, or 0.6/F
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.
SW
. Put simply,
SW
.