Datasheet

ON
OUTIN
peakLPSM
t
L
VV
I ×
-
=
)(
)(
s
V
V
t
IN
OUT
ON
m1×=
TPS62175, TPS62177
www.ti.com SLVSB35B –OCTOBER 2012–REVISED JANUARY 2014
DETAILED DESCRIPTION
Device Operation
The TPS62175/7 synchronous switch mode power converters are based on DCS-Control™ (Direct Control with
Seamless Transition into Power Save Mode), an advanced regulation topology, that combines the advantages of
hysteretic, voltage mode and current mode control including an AC loop directly associated to the output voltage.
This control loop takes information about output voltage changes and feeds it directly to a fast comparator stage.
It sets the switching frequency, which is constant for steady state operating conditions, and provides immediate
response to dynamic load changes. To get accurate DC load regulation, a voltage feedback loop is used. The
internally compensated regulation network achieves fast and stable operation with small external components
and low ESR capacitors. The DCS-Control
TM
topology supports PWM (Pulse Width Modulation) mode for
medium and heavy load conditions and a Power Save Mode at light loads. During PWM, it operates at its
nominal switching frequency in continuous conduction mode. This frequency is typically about 1MHz with a
controlled frequency variation depending on the input voltage. If the load current decreases, the converter enters
Power Save Mode to sustain high efficiency down to very light loads. In Power Save Mode the switching
frequency decreases linearly with the load current. Since DCS-Control™ supports both operation modes within
one single building block, the transition from PWM to Power Save Mode is seamless without effects on the
output voltage. Fixed output voltage versions provide smallest solution size and lowest current consumption,
requiring only 3 external components. An internal current limit supports nominal output currents of up to 500mA.
The TPS62175/7 offer both excellent DC voltage and superior load transient regulation, combined with very low
output voltage ripple, minimizing interference with RF circuits.
Pulse Width Modulation (PWM) Operation
The TPS62175/7 operates with pulse width modulation in continuous conduction mode (CCM) with a nominal
switching frequency of about 1MHz. The switching frequency in PWM is set by an internal timer circuit. The
frequency variation is controlled and depends on V
IN
and V
OUT
. The device operates in PWM mode as long the
output current is higher than half the inductor's ripple current. To maintain high efficiency at light loads, the
device enters Power Save Mode at the boundary to discontinuous conduction mode (DCM).
Power Save Mode Operation
The TPS62175/7's built in Power Save Mode is entered seamlessly, if the load current decreases. This secures a
high efficiency in light load operation by keeping the on-time fixed and reducing the switching frequency by
incorporating pause time. The device remains in Power Save Mode as long as the inductor current is
discontinuous. The on-time, in steady-state PWM operation, can be estimated as:
spacing
(1)
spacing
In case equation (1) yields a lower value, the device maintain an on-time of about 80ns to limit switching losses.
This minimum on-time is used in Power Save Mode. While the peak inductor current in Power Save Mode can be
approximated by:
spacing
(2)
spacing
The switching frequency is calculated as follows:
spacing
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