Datasheet

( )
IN OUT
LPFMpeak ON
V V
I = T
L
-
´
s
V
V
T
IN
OUT
ON
m1´=
TPS62125
SLVSAQ5C –MARCH 2012–REVISED DECEMBER 2013 www.ti.com
DETAILED DESCRIPTION
DCS - Control
TM
The TPS62125 high efficiency synchronous switch mode buck converter includes TI's DCS - Control™ (Direct
Control with Seamless Transition into Power Save Mode), an advanced regulation topology, which combines the
advantages of hysteretic and voltage mode control. Characteristics of DCS - Control
TM
are excellent AC load
regulation and transient response, low output ripple voltage and a seamless transition between PFM and PWM
mode operation.
DCS - Control
TM
includes an AC loop which senses the output voltage (VOS pin) and directly feeds the
information to a fast comparator stage. This comparator sets the switching frequency, which is constant for
steady state operating conditions, and provides immediate response to dynamic load changes. In order to
achieve 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 high load conditions and a
Power Save Mode at light loads. During PWM mode, it operates in continuous conduction. The switch frequency
is up to 1MHz with a controlled frequency variation depending on the input voltage. If the load current decreases,
the converter seamless enters Power Save Mode to maintain high efficiency down to very light loads. In Power
Save Mode the switching frequency varies linearly with the load current. Since DCS - Control
TM
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. The TPS62125 offers both excellent DC voltage and superior load transient
regulation, combined with very low output voltage ripple, minimizing interference with RF circuits.
At high load currents the converter operates in quasi fixed frequency PWM mode operation and at light loads in
PFM (Pulse Frequency Modulation) mode to maintain highest efficiency over the full load current range. In PFM
Mode, the device generates a single switching pulse to ramp up the inductor current and recharge the output
capacitor, followed by a sleep period where most of the internal circuits are shutdown to achieve a quiescent
current of typically 13µA. During this time, the load current is supported by the output capacitor. The duration of
the sleep period depends on the load current and the inductor peak current.
Pulse Width Modulation (PWM) Operation
The TPS62125 operates with pulse width modulation in continuous conduction mode (CCM) with a nominal
switching frequency of about 1MHz. The frequency variation in PWM mode is controlled and depends on VIN,
VOUT and the inductance. 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). This happens if the output current becomes smaller than
half the inductor's ripple current.
Power Save Mode
With decreasing load current, the TPS62125 transitions seamlessly from PWM Mode to Power Save Mode once
the inductor current becomes discontinuous. This ensures a high efficiency at light loads. In Power Save Mode
the converter operates in Pulse Frequency Modulation (PFM Mode) and the switching frequency decreases
linearly with the load current. DCS - Control
TM
features a small and predictable output voltage ripple in Power
Save Mode. The transition between PWM Mode and Power Save Mode occurs seamlessly in both directions.
The minimum On Time T
ONmin
for a single pulse can be estimated by:
(1)
Therefore the peak inductor current in PFM mode is approximately:
(2)
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