LP3942 datasheet(17/20 Pages) NSC

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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 0

Ω,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

DD value is reflected to VOUT.

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

IN) be placed as

close as possible to the LP3942, so that the trace from the

input pin (V

IN) is as short and straight as possible. It is

recommended that the input capacitor (C

IN) is placed on the

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

OUT) be placed as close to the LP3942 output

pad (P

OUT) as possible. It is best if routing of output pad

trace follows guidelines similar to those presented for the

input pad (V

IN) and capacitor (CIN). The flying capacitors (C1

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.

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

Ω 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 M

Ω 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

OUT) of the charge pump. The expected voltage

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

OUT).

Output resistance models the output voltage droop that is

inherent to switched capacitor converters. The output resis-

tance of the LP3942 is 5

Ω (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

OUT increases. To

prevent droop in output voltage, the voltage drop across the

regulator is reduced, V’ increases, and V

OUT remains at

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-

20129215

FIGURE 1. Charge Pump Output Resistance Model

LP3942 Datasheet(PDF) 17 Page - National Semiconductor (TI)

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