DETAILED DESCRIPTION

OPERATION

Power Save Mode

TPS62270

TPS62273

The TPS62270 step down converter operates with typically 2.25 MHz fixed frequency pulse width modulation

(PWM) at moderate to heavy load currents. At light load currents the converter automatically enters Power Save

Mode and operates then in PFM mode.

During PWM operation the converter use a unique fast response voltage mode controller scheme with input

voltage feed-forward to achieve good line and load regulation allowing the use of small ceramic input and output

capacitors. At the beginning of each clock cycle initiated by the clock signal, the High Side MOSFET switch is

turned on. The current flows now from the input capacitor via the High Side MOSFET switch through the inductor

to the output capacitor and load. During this phase, the current ramps up until the PWM comparator trips and the

control logic will turn off the switch. The current limit comparator will also turn off the switch in case the current

limit of the High Side MOSFET switch is exceeded. After a dead time preventing shoot through current, the Low

Side MOSFET rectifier is turned on and the inductor current will ramp down. The current flows now from the

inductor to the output capacitor and to the load. It returns back to the inductor through the Low Side MOSFET

rectifier.

The next cycle will be initiated by the clock signal again turning off the Low Side MOSFET rectifier and turning on

the on the High Side MOSFET switch.

If the load current decreases, the converter will enter Power Save Mode operation automatically. During Power

Save Mode the converter skips switching and operates with reduced frequency in PFM mode with a minimum

quiescent current to maintain high efficiency.

The transition from PWM mode to PFM mode occurs once the inductor current in the Low Side MOSFET switch

becomes zero, which indicates discontinuous conduction mode.

During the Power Save Mode the output voltage is monitored with a PFM comparator. As the output voltage falls

below the PFM comparator threshold of VOUT +1%, the device starts a PFM current pulse. For this the High

Side MOSFET switch will turn on and the inductor current ramps up. After the On-time expires the switch will be

turned off and the Low Side MOSFET switch will be turned on until the inductor current becomes zero.

The converter effectively delivers a current to the output capacitor and the load. If the load is below the delivered

current the output voltage will rise. If the output voltage is equal or higher than the PFM comparator threshold,

the device stops switching and enters a sleep mode with typical 15

µA current consumption.

In case the output voltage is still below the PFM comparator threshold, further PFM current pulses will be

generated until the PFM comparator threshold is reached. The converter starts switching again once the output

voltage drops below the PFM comparator threshold.

With a fast single threshold comparator, the output voltage ripple during PFM mode operation can be kept very

small. The PFM Pulse is timing controlled, which allows to modify the charge transferred to the output capacitor

by the value of the inductor. The resulting PFM output voltage ripple depends in first order on the size of the

output capacitor and the inductor value. Increasing output capacitor values and/or inductor values will minimize

the output ripple.

The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM

mode.

Copyright © 2007–2008, Texas Instruments Incorporated

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Product Folder Link(s): TPS62270 TPS62273