( )

(

)

-

´

=

´

´ ´

IN

OUT

OUT

OUT LL

SW

IN

V

V

V

1

I

2

L

f

V

TPS53353

SLUSAK2B – AUGUST 2011 – REVISED FEBRUARY 2015

www.ti.com

Device Functional Modes (continued)

chemistry. For example, specialty polymer capacitors (SP-CAP) have an output capacitance in the order of

several 100 µF and ESR in range of 10 m

modulator. An application circuit for ceramic capacitor is described in the External Component Selection Using

All Ceramic Output Capacitors section.

7.4.2 Enable, Soft Start, and Mode Selection

When the EN pin voltage rises above the enable threshold voltage (typically 1.2 V), the controller enters its start-

up sequence. The internal LDO regulator starts immediately and regulates to 5 V at the VREG pin. The controller

then uses the first 250

μs to calibrate the switching frequency setting resistance attached to the RF pin and

stores the switching frequency code in internal registers. During this period, the MODE pin also senses the

resistance attached to this pin and determines the soft-start time. Switching is inhibited during this phase. In the

second phase, an internal DAC starts ramping up the reference voltage from 0 V to 0.6 V. Depending on the

MODE pin setting, the ramping up time varies from 0.7 ms to 5.6 ms. Smooth and constant ramp-up of the

output voltage is maintained during start-up regardless of load current.

Table 3. Soft-Start and MODE Settings

MODE SELECTION

ACTION

SOFT-START TIME (ms)

0.7

39

1.4

100

Auto-skip

Pulldown to GND

2.8

200

5.6

475

0.7

39

1.4

100

Connect to PGOOD

2.8

200

5.6

475

(1)

Device enters FCCM after the PGOOD pin goes high when MODE is connected to PGOOD through

After soft-start begins, the MODE pin becomes the input of an internal comparator which determines auto-skip or

FCCM mode operation. If MODE voltage is higher than 1.3 V, the converter enters into FCCM mode. Otherwise

it will be in auto-skip mode at light load condition. Typically, when FCCM mode is selected, the MODE pin is

auto-skip mode.

7.4.3 Auto-Skip Eco-mode™ Light Load Operation

load conditions to maintain high efficiency. Detailed operation is described as follows. As the output current

decreases from heavy load condition, the inductor current is also reduced and eventually comes to the point that

its rippled valley touches zero level, which is the boundary between continuous conduction and discontinuous

conduction modes. The synchronous MOSFET is turned off when this zero inductor current is detected. As the

load current further decreases, the converter runs into discontinuous conduction mode (DCM). The on-time is

kept almost the same as it was in the continuous conduction mode so that it takes longer to discharge the output

capacitor with smaller load current to the level of the reference voltage. The transition point to the light-load

calculated as shown in Equation 7.

where

•

(7)

18

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