Datasheet

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   
SLVS216B − JUNE 1999 − JUNE 2008
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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
output voltage V
O
reaches 0.8×V
IN
. When the start-up comparator detects this limit, the IC begins to operate
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
When SKIP is low, the charge pump runs continuously at the frequency f
OSC
. The control circuit, fed from the
error amplifier, controls the charge on C
1F
and C
2F
by driving the gates of the FETs T
12
/T
13
and T
22
/T
23
,
respectively. When the output voltage falls, the gate drive increases, resulting in a larger voltage across C
1F
and C
2F
. This regulation scheme minimizes output ripple. Since the device switches continuously, the output
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
C
IN
4.7 µF
OUTPUT
5 V 150 mA
C
O
= 33 µF
C
2F
1 µF
C
1F
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.