LP3942YQ NSC [National Semiconductor], LP3942YQ Datasheet - Page 19

• Manufacturer Part Number: LP3942YQ
• Description: The Dual RGB LED Controller with 1.5x/2x Charge Pump and SPI Interface
• Manufacturer: NSC [National Semiconductor]

Charge Pump Application
Information
have a capacitance of only 0.1 µF. Such detrimental devia-
tion is likely to cause these Y5V and Z5U of capacitors to fail
to meet the minimum capacitance requirements of the
LP3942. The table below lists some leading ceramic capaci-
tor manufacturers.
INPUT CAPACITORS
The input capacitor (CIN) 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 60 mA and 120 mA, a minimum input capacitor of
2.2 µF is required. For applications where the maximum load
current is 60 mA or less, 1.0 µF of input capacitor is suffi-
cient. Failure to provide enough capacitance on the LP3942
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 (C1 and C2)
The flying capacitor (CFLY) transfers charge from the input
to the output, providing the voltage boost of the charge
pump. A polarized capacitor (tantalum, aluminum electro-
lytic, etc.) must not be used here, as the capacitor will be
reverse-biased upon start-up of the LP3942. The size of the
flying capacitor and its ESR affect output current capability
when the input voltage of the LP3942 is low, most notable for
input voltages below 3.4V. These issues were discussed
previously in the Output Current Capability section. For
most applications, a 1 µF X7R/X5R ceramic capacitor is
recommended for the flying capacitor. When considering
also voltage roll-off, minimum capacitance value of 700 nF
should be available in all voltage conditions.
OUTPUT CAPACITOR
The output capacitor of the LP3942 plays an important part
in determining the characteristics of the output signal of the
LP3942, 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 ce-
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Contact Information
19
ramic capacitor is the capacitor of choice for the LP3942
output. In addition to these issues previously discussed, the
output capacitor of the LP3942 also affects control-loop sta-
bility 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. Minimum output capaci-
tance of 3.0 µF is required.
POWER EFFICIENCY AND POWER DISSIPATION ON
1.5x MODE
Efficiency of the LP3942 mirrors that of an unregulated
switched capacitor converter followed by a linear regulator.
The simplified power model of the LP3942, in Figure 2, will
be used to discuss power efficiency and power dissipation.
In calculating power efficiency, output power (P
determined as the product of the output current and the 4.5V
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 3/2-multiplier must be 1.5 times the
output current. Adding a correction factor for operating qui-
escent current (IQ, 1.2 mA 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:
Because efficiency is inversely proportional to input voltage,
it is highest when the input voltage is low. In fact, for an input
voltage of 3.4V, efficiency of the LP3942 is greater than 80%
(I
range spanning the input voltage range (3.4V-to-4.2V) is
75% (I
drops dramatically. In Li-Ion powered 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.
LP3942’s charge pump power dissipation (PD) is calculated
simply by subtracting output power from input power:
Power dissipation increases with increased input voltage
and output current, up to 450 mW at the ends of the oper-
ating ratings (V
dissipation self-heats the device. Dissipating this amount
power/ heat so the LP3942 does not overheat is a demand-
ing 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 pack-
age enable this power to be dissipated from the LP3942 with
little or no derating, even when the circuit is placed in el-
evated ambient temperatures.
P
OUT
D
= P
≥ 40 mA). The average efficiency for an input voltage
out
IN
= 120 mA). At higher input voltages, efficiency
– P
OUT
IN
= 5.5V, I
= [V
IN
x (1.5·I
OUT
= 120 mA). Internal power
OUT
+ I
Q
)] – [V
OUT
OUT
www.national.com
) is easily
x I
OUT
]