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DETAILED DESCRIPTION

PWM OPERATION

V

OUT [

1 *

R

1

R

2

0.75 V

(1)

LIGHT LOAD CONDITION WITH AUTO-SKIP FUNCTION

I

OUT(LL) [

1

2

L

ƒsw

V

IN *

V

OUT

V

OUT

V

IN

(2)

TPS51117

SLVS631 – DECEMBER 2005

The main control loop of the TPS51117 is designed as an adaptive on-time pulse width modulation (PWM)

controller. It supports proprietary D-CAP™ Mode that uses an internal compensation circuit and is suitable for

minimal external component count configuration when an appropriate amount of ESR at the output capacitor(s) is

allowed. Basic operation of D-CAP Mode can be described as follows.

At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This

MOSFET is turned off, or becomes OFF state, after the internal one shot timer expires. This one shot is

hence it is called adaptive on-time control or fixed frequency emulated on-time control (see PWM frequency and

voltage, indicates insufficient output voltage AND inductor current information indicates below the overcurrent

limit. Repeating the operation in this manner, the controller regulates the output voltage. The synchronous

low-side or rectifying MOSFET is turned on each OFF state to keep the conduction loss to a minimum.

The TPS51117 supports selectable PWM-only and auto-skip operation modes. If EN_PSV is grounded, the

switching regulator is disabled. If the EN_PSV pin is connected to 3.3 V or 5 V, the regulator is enabled with

auto-skip mode selected. The rectifying MOSFET is turned off when inductor current information detects zero

level. This enables a seamless transition to reduced frequency operation during a light load condition so that high

efficiency is maintained over a broad range of load currents. If the EN_PSV pin is floated, it is internally pulled up

to 1.95 V, and the regulator is enabled with PWM-only mode selected. The rectifying MOSFET is not turned off

when inductor current reaches zero. The converter runs forced continuous conduction mode for the entire load

range. System designers may want to use this mode to avoid a certain frequency during a light load condition but

with the cost of low efficiency. However, be aware the output has the capability to both source and sink current in

this mode. If the output terminal is connected to a voltage source higher than the regulator’s target, the converter

sinks current from the output and boosts the charge into the input capacitor. This may cause unexpected high

voltage at VIN and may damage the power FETs.

DC output voltage can be set by the external resistor divider as follows (refer to Figure 22 and Figure 23).

If auto-skip mode is selected, the TPS51117 automatically reduces the switching frequency during a light load

condition to maintain high efficiency. This reduction of frequency is achieved smoothly and without an increase of

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

touches zero current, which is the boundary between continuous conduction and discontinuous conduction

modes. The rectifying MOSFET is turned off when this zero inductor current is detected. Since the output voltage

is still higher than the reference at this moment, both high-side and low-side MOSFETs are turned off and wait

for the next cycle. As the load current decreases further, the converter runs in discontinuous conduction mode,

taking longer time to discharge the output capacitor below the reference voltage. Note the ON time is kept the

same as during the heavy load condition. In reverse, when the output current increases from a light load to a

heavy load, the switching frequency increases to the preset value as the inductor current reaches to the

continuous and discontinuous conduction mode), can be calculated as follows:

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