LP3942YQ NSC [National Semiconductor], LP3942YQ Datasheet - Page 17

• 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 Operational
Description
OVERVIEW
The LP3942 includes a regulated switched-capacitor charge
pump with two programmable voltage multiplications, 1.5
and 2.
On 1.5x mode by combining the principles of a switched-
capacitor charge pump and a linear regulator, it generates a
regulated 4.5V output from Li-Ion input voltage range. A
two-phase internally generated non-overlapping clock con-
trols the operation of the charge pump. During the charge
phase (ø1), both flying capacitors (C1 and C2) are charged
from input voltage. In the pump phase that follows (ø2), the
flying capacitors are discharged to output. A traditional
switched capacitor charge pump operating in this manner
will use switches with very low on-resistance, ideally 0Ω, to
generate an output voltage that is 1.5x the input voltage. The
LP3942 regulates the output voltage by controlling the resis-
tance of the input-connected pass-transistor switches in the
charge pump. On 2x mode the output is regulated to 5.0V
thus enabling the higher output voltage for driving parallel
connected RGBs with lower current variation on RGB switch
mode.
PRE-REGULATION
The low input current ripple of the LP3942, resulting from
internal pre-regulation, adds very little noise to the input line.
Regulation is achieved by modulating the on-resistance of
the switches connected to the input pin. The regulation is
done before the voltage multiplication, giving rise to the term
“pre-regulation”. It is pre-regulation that eliminates most of
the input current ripple that is a typical and undesirable
characteristic of a many switched capacitor converters. V
is used as the regulator reference voltage. Any change in
V
INPUT, OUTPUT, AND GROUND CONNECTIONS
Making good input, output, and ground connections is es-
sential to achieve optimal LP3942 performance. It is strongly
recommended that the input capacitor (C
close as possible to the LP3942, so that the trace from the
input pin (V
recommended that the input capacitor (C
same side of the PCB as LP3942, and that traces remain on
this side of the board as well (vias to traces on other PCB
layers are not recommended between the input capacitor
and LP3942 input pad). It is recommended that the output
capacitor (C
pad (P
trace follows guidelines similar to those presented for the
input pad (V
and C2) should also be placed as close to the LP3942 as
possible to minimize PCB trace length between the capacitor
and the IC.
The following pads of the LP3942 are ground connections
and must be connected externally: pads GND1, GND2,
GND3 and the die-attach pad (DAP). Large, low impedance
copper fills and via connections to an internal ground plane
are the preferred way of connecting together the ground
pads of the LP3942, the input capacitor, and the output
capacitor, as well as connecting this circuit ground to the
system ground of the PCB.
DD
value is reflected to V
OUT
) as possible. It is best if routing of output pad
IN
OUT
IN
) and capacitor (C
) is as short and straight as possible. It is
) be placed as close to the LP3942 output
OUT
.
IN
). The flying capacitors (C1
IN
) is placed on the
IN
) be placed as
DD
17
RESET AND STANDBY
When the voltage on the NRST pin is high, the LP3942 will
be in reset mode. After NRST goes low, the LP3942 goes to
default mode, which is Standby. All internal registers in
LP3942 are set to default state in reset mode. There is a 1
MΩ pull-down resistor tied between the NRST pin and
ground that pulls the NRST pin voltage low if the pin is not
driven by a voltage source. When pulling the part out of reset
mode, the voltage source connected to the NRST pin must
be able to drive the current required by the 1 MΩ resistor.
SOFT START
The LP3942 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
(4.5V or 5.0V) in 200 µs (typ). Soft-start is engaged after the
specified start-up delay (0.8 ms typically) after a part, with
input voltage established, is taken out of standby by writing
NSTBY and CP_ON bits to ‘1’. Start-up delay and soft-start
will also engage always when CP_ON bit is written to ‘1’ or
when device recovers from thermal shutdown mode.
OUTPUT CURRENT CAPABILITY
In 1.5x mode the LP3942 is guaranteed to provide 120 mA of
output current at specified output voltage when the input
voltage is within 3.4V-to-5.0V. LP3942 can provide 120 mA
current also from lower input voltage (down to 3.0V) but then
output voltage will be degraded due to effective output resis-
tance (R
drop can be calculated by using a simple model for the
charge pump depicted in Figure 1.
The model shows a linear pre-regulation block (Reg), a
voltage multiplier (1.5x), and an output resistance (R
Output resistance models the output voltage droop that is
inherent to switched capacitor converters. The output resis-
tance of the LP3942 is 5Ω (typ), and is function of switching
frequency, flying capacitors, internal resistances of switches
and ESR of capacitors. When the output voltage is in regu-
lation, the regulator in the model controls the voltage V’ to
keep the output voltage equal to 4.5V (typ). With increased
output current, the voltage drop across R
prevent droop in output voltage, the voltage drop across the
regulator is reduced, V’ increases, and V
4.5V. When the output current 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 volt-
age to fall out of regulation, and the LP3942 operation is
similar to a basic open-loop 1.5x charge pump. In this mode,
output current results in output voltage drop proportional to
the output resistance of the charge pump. The out-of-
FIGURE 1. Charge Pump Output Resistance Model
OUT
) of the charge pump. The expected voltage
OUT
OUT
increases. To
www.national.com
remains at
20129215
OUT
).