TPS62350 datasheet(21/46 Pages) TI

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DETAILED DESCRIPTION

Operation

Power-Save Mode : Fast PFM

PFMModeatLightLoad

PFMRipple

PWMModeatHeavyLoad

CompLowThreshold=V NOM

TPS62350, TPS62351

TPS62352, TPS62353, TPS62354

SLVS540B – MAY 2006 – REVISED DECEMBER 2006

The TPS6235x is a synchronous step-down converter typically operating with a 3-MHz fixed frequency pulse

width modulation (PWM) at moderate to heavy load currents. At light load currents, the converter operates in

power-save mode with pulse frequency modulation (PFM). The device integrates two power-save modes

optimized either for ultra-high efficiency at light load (light PFM) or for transient response when turning in PWM

operation (fast PFM). Both power-save modes automatically transition to PWM operation when the load current

increases.

The TPS6235x integrates an I2C compatible interface allowing transfers up to 3.4 Mbps. This communication

interface can be used for dynamic voltage scaling with voltage steps down to 12.5 mV, for reprogramming the

mode of operation (light PFM, fast PFM or forced PWM) or disable/enabling the output voltage for instance. For

more details, refer to the I2C interface and register description section.

During PWM operation, the converter uses a unique fast response, voltage mode, control scheme with input

voltage feed-forward. This achieves best-in-class load and line response and allows the use of tiny inductors

and small ceramic input and output capacitors. At the beginning of each switching cycle, the P-channel

MOSFET switch is turned on and the inductor current ramps up until the comparator trips and the control logic

turns off the switch. The operating frequency is set to 3 MHz and can be synchronized on-the-fly to an external

oscillator or to a master dc/dc converter (refer to application examples).

The device integrates two current limits, one in the P-channel MOSFET and another one in the N-channel

MOSFET. When the current in the P-channel MOSFET reaches its current limit, the P-channel MOSFET is

turned off and the N-channel MOSFET is turned on. When the current in the N-channel MOSFET is above the

N-MOS current limit threshold, the N-channel MOSFET remains on until the current drops below its current limit.

The current limit in the N-channel MOSFET is important for small duty-cycle operation when the current in the

inductor does not decrease because of the P-channel MOSFET current limit delay, or because of start-up

conditions where the output voltage is low.

With decreasing load current, the device automatically switches into pulse skipping operation in which the power

stage operates intermittently based on load demand. By running cycles periodically, the switching losses are

minimized, and the device runs with a minimum quiescent current and maintains high efficiency.

In fast PFM mode, the converter only operates when the output voltage trips below a set threshold voltage (VO

nominal). It ramps up the output voltage with several pulses and goes into power-save mode when the inductor

current reaches zero. As a consequence in power-save mode the average output voltage is slightly higher than

its nominal value in PWM mode. The fast PFM mode is optimized for fast response when transitioning between

pulse skipping and PWM operation.

Figure 44. Operation in PFM Mode and Transfer to PWM Mode

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