SLVS848A – JULY 2009 – REVISED JULY 2009 ............................................................................................................................................................... www.ti.com
The TPS6262x is a synchronous step-down converter typically operates at a regulated 6-MHz frequency pulse
width modulation (PWM) at moderate to heavy load currents. At light load currents, the TPS6262x converter
operates in power-save mode with pulse frequency modulation (PFM).
The converter uses a unique frequency locked ring oscillating modulator to achieve 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 rising the
output voltage until the main comparator trips, then the control logic turns off the switch.
One key advantage of the non-linear architecture is that there is no traditional feed-back loop. The loop response
to change in VO is essentially instantaneous, which explains the transient response. The absence of a traditional,
high-gain compensated linear loop means that the TPS6262x is inherently stable over a range of L and CO.
Although this type of operation normally results in a switching frequency that varies with input voltage and load
current, an internal frequency lock loop (FLL) holds the switching frequency constant over a large range of
Combined with best in class load and line transient response characteristics, the low quiescent current of the
device (ca. 31
µA) allows to maintain high efficiency at light load, while preserving fast transient response for
applications requiring tight output regulation.
The magnitude of the internal ramp, which is generated from the duty cycle, reduces for duty cycles either set of
50%. Thus, there is less overdrive on the main comparator inputs which tends to slow the conversion down. The
intrinsic maximum operating frequency of the converter is about 10MHz to 12MHz, which is controlled to circa.
6MHz by a frequency locked loop.
When high or low duty cycles are encountered, the loop runs out of range and the conversion frequency falls
below 6MHz. The tendency is for the converter to operate more towards a "constant inductor peak current" rather
than a "constant frequency". In addition to this behavior which is observed at high duty cycles, it is also noted at
low duty cycles.
When the converter is required to operate towards the 6MHz nominal at extreme duty cycles, the application can
be assisted by decreasing the ratio of inductance (L) to the output capacitor's equivalent serial inductance (ESL).
This increases the ESL step seen at the main comparator's feed-back input thus decreasing its propagation
delay, hence increasing the switching frequency.
If the load current decreases, the converter will enter Power Save Mode operation automatically. During
power-save mode the converter operates in discontinuous current (DCM) single-pulse PFM mode, which
produces low output ripple compared with other PFM architectures.
When in power-save mode, the converter resumes its operation when the output voltage trips below the nominal
voltage. It ramps up the output voltage with a minimum of one pulse and goes into power-save mode when the
inductor current has returned to a zero steady state. The PFM on-time varies inversely proportional to the input
voltage and proportional to the output voltage giving the regulated switching frequency when in steady-state.
PFM mode is left and PWM operation is entered as the output current can no longer be supported in PFM mode.
As a consequence, the DC output voltage is typically positioned ca. 0.5% above the nominal output voltage and