TPS60111

REGULATED 5V 150mA LOW NOISE

CHARGE PUMP DC/DC CONVERTER

SLVS216B − JUNE 1999 − JUNE 2008

11

POST OFFICE BOX 655303

detailed description (continued)

start-up procedure

During start-up, i.e. when ENABLE is set from logic low to logic high, the switches T12 and T14 (charge pump

1), and the switches T22 and T24 (charge pump 2) are conducting to charge up the output capacitor until the

in the mode selected with SKIP and COM. This start-up charging of the output capacitor guarantees a short

start-up time and eliminates the need for a Schottky diode between IN and OUT.

pulse-skip mode

In pulse-skip mode (SKIP = high), the error amplifier disables switching of the power stages when it detects an

output higher than 5 V. The oscillator halts. The IC then skips switching cycles until the output voltage drops

below 5 V. Then the error amplifier reactivates the oscillator and switching of the power stages starts again. The

pulse-skip regulation mode minimizes operating current because it does not switch continuously and

deactivates all functions except bandgap reference and error amplifier when the output is higher than 5 V. When

switching is disabled from the error amplifier, the load is also isolated from the input. SKIP is a logic input and

should not remain floating. The typical operating circuit of the TPS60111 in pulse skip mode is shown in Figure

1.

constant-frequency mode

noise contains well-defined frequency components, and the circuit requires smaller external capacitors for a

given output ripple. However, constant-frequency mode, due to higher operating current, is less efficient at light

loads than pulse-skip mode.

IN

IN

C1+

C1−

ENABLE

OUT

OUT

FB

C2+

C2−

SYNC

SKIP COM CLK

PGND GND

INPUT

2.7 V to 5.4 V

CIN

4.7

µF

OUTPUT

5 V 150 mA

CO = 33 µF

C2F

1

µF

C1F

1

µF

+

OFF/ON

TPS60111

+

Figure 26. Typical Operating Circuit TPS60111 in Constant Frequency Mode

Table 1. Tradeoffs Between Operating Modes

FEATURE

PULSE-SKIP MODE

(SKIP = High)

CONSTANT-FREQUENCY MODE

(SKIP = Low)

Best light-load efficiency

X

Smallest external component size for a given output ripple

X

Output ripple amplitude

Small amplitude

Very small amplitude

Output ripple frequency

Variable

Constant

Load regulation

Very good

Good

NOTE: Even in pulse-skip mode the output ripple amplitude is small if the push-pull operating mode is selected via COM.