LM2767 datasheet(10/21 Pages) TI

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POUT

L + IL

OUT + IQ (V+)

RIPPLE

&OSC × C2

+ 2 × I

L × ESRC2

OUT

SW +

&OSC × C1

+ 4ESR

C1 + ESRC2

LM2767

SNVS069D – FEBRUARY 2000 – REVISED AUGUST 2015

www.ti.com

The output characteristics of this circuit can be approximated by an ideal voltage source in series with a

resistance. The voltage source equals 2 V+. The output resistance Rout is a function of the ON resistance of the

internal MOSFET switches, the oscillator frequency, and the capacitance and ESR of C1 and C2. Because the

switching current charging and discharging C1 is approximately twice the output current, the effect of the ESR of

the pumping capacitor C1 is multiplied by four in the output resistance. The output capacitor C2 is charging and

discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the

output resistance. A good approximation of Rout is:

where

•

RSW is the sum of the ON resistance of the internal MOSFET switches shown in Figure 11.

(1)

The peak-to-peak output voltage ripple is determined by the oscillator frequency as well as the capacitance and

ESR of the output capacitor C2:

(2)

High capacitance, low ESR capacitors can reduce both the output resistance and the voltage ripple.

The Schottky diode D1 is only needed to protect the device from turning on its own parasitic diode and potentially

latching up. During start-up, D1 also quickly charges up the output capacitor to VIN minus the diode drop thereby

decreasing the start-up time. Therefore, the Schottky diode D1 must have enough current carrying capability to

charge the output capacitor at start-up, as well as a low forward voltage to prevent the internal parasitic diode

from turning on. A Schottky diode like 1N5817 can be used for most applications. If the input voltage ramp is less

than 10 V/ms, a smaller Schottky diode like MBR0520LT1 can be used to reduce the circuit size.

9.2.2.2 Capacitor Selection

As discussed in Positive Voltage Doubler, the output resistance and ripple voltage are dependent on the

capacitance and ESR values of the external capacitors. The output voltage drop is the load current times the

output resistance, and the power efficiency is

where

•

IQ(V+) is the quiescent power loss of the device; and

•

out is the conversion loss associated with the switch on-resistance, the two external capacitors and their

ESRs.

(3)

The selection of capacitors is based on the allowable voltage droop (which equals Iout Rout), and the desired

output voltage ripple. Low-ESR capacitors (Table 2) are recommended to maximize efficiency, reduce the output

voltage drop and voltage ripple.

Table 2. Low-ESR Capacitor Manufacturers

MANUFACTURER

PHONE

WEBSITE

CAPACITOR TYPE

Nichicon Corp.

(847)-843-7500

www.nichicon.com

PL & PF series, through-hole aluminum electrolytic

AVX Corp.

(843)-448-9411

www.avxcorp.com

TPS series, surface-mount tantalum

Sprague

(207)-324-4140

www.vishay.com

593D, 594D, 595D series, surface-mount tantalum

Sanyo

(619)-661-6835

www.sanyovideo.com

OS-CON series, through-hole aluminum electrolytic

Murata

(800)-831-9172

www.murata.com

Ceramic chip capacitors

Taiyo Yuden

(800)-348-2496

www.t-yuden.com

Ceramic chip capacitors

Tokin

(408)-432-8020

www.tokin.com

Ceramic chip capacitors

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