Datasheet

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Adaptive On-Time Control

Programming Table

TPS51120

SLUS670B – JULY 2005 – REVISED FEBRUARY 2007

DETAILED DESCRIPTION (continued)

The TPS51120 employs adaptive on time control scheme and does not have a dedicated oscillator on board.

However, it works almost constant frequency over the entire input voltage range (pseudo-constant frequency) by

feed-forwarding the input and output voltage into the on-time one-shot timer. The on-time is controlled inverse

proportional to the input voltage and proportional to the output voltage so that the duty ratio is kept as V

OUT

technically with the same cycle time. The input voltage monitoring is accomplished through sensing the LL node,

not at VIN node, during the ‘ON’ state. This eliminates the influence of the voltage drop across the top MOSFET

to the frequency especially in heavy load condition. The VIN pin is not used for the on-time control but used only

for the 5 V and 3.3 V regulators’ supply. The switching frequency is selectable from four combinations shown in

the table below by setting TONSEL pin voltage. This allows the system design to pursue highest efficiency (5

V/180 kHz, 3.3 V/270 kHz), smallest components size (5 V/380 kHz, 3.3 V/580 kHz) or a good balance of both in

the medium. Also shown in the table are the typical on-time for each frequency and 5 V, 3.3 V outputs at

VIN=12. Output voltage feed-forward is enabled after the output voltage exceeds 1.0 V in order to achieve stable

start up.

Table 1. Frequency Selection and Typical On-Time

TONSEL CONNECTION CH1(LL1=VIN=12 V) CH2 (LL2=VIN=12 V)

FREQUENCY (kHz) ON-TIME @ 5 V (ns) FREQUENCY (kHz) ON-TIME @ 3.3 V (ns)

V5FILT 180 2340 270 1030

FLOAT (OPEN) 220 1950 330 850

VREF2 280 1490 430 650

GND 380 1111 580 480

The TPS51120 has varieties of configurations choice. It is important to tailor appropriately with regard to the

system design requirements. Table below shows programming table for the control scheme selection, frequency

selection, output voltage selection and skip selection. Faults-off disables UVP, OVP and UVLO. This is mainly

intended for debugging purpose. Enable states and possible connections for the LDO’s EN3, EN5 pins and

SMPS’s EN1, EN2 pins are also shown.

Table 2. Function Programming Table

PIN GND VREF2 FLOAT V5FILT

COMP N/A N/A Current Mode D-CAP™ Mode

(apply R-C network)

TONSEL (CH1/CH2) [kHz] 380 / 580 280 / 430 220 / 330 180 / 270

VFB1 Adjustable output (connect to the resistor divider) 5V fixed output

VFB2 Adjustable output (connect to the resistor divider) 3.3 V fixed output

SKIPSEL AUTO-SKIP AUTO-SKIP (FAULTS OFF) PWM PWM

EN1, EN2 Switcher Off Not used Switcher on Switcher on

EN3, EN5 LDO Off Not used LDO on LDO on (EN3 only)

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