TPS56528
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SLVSBV3A –APRIL 2013–REVISED APRIL 2013
OVERVIEW
The TPS56528 is a 5-A synchronous step-down (buck) converter with two integrated N-channel MOSFETs. It
operates using D-CAP2™ mode control. The fast transient response of D-CAP2™ control reduces the output
capacitance required to meet a specific level of performance. Proprietary internal circuitry allows the use of low
ESR output capacitors including ceramic and special polymer types. And also PG output can be used for
sequence operation.
DETAILED DESCRIPTION
PWM Operation
The main control loop of the TPS56528 is an adaptive on-time pulse width modulation (PWM) controller that
supports a proprietary D-CAP2™ mode control. D-CAP2™ mode control combines constant on-time control with
an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with
both low ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output.
At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one
shot timer expires. This one shot is set by the converter input voltage, VIN, and the output voltage, VO, to
maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The
one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the
reference voltage. An internal ramp is added to reference voltage to simulate output ripple, eliminating the need
for ESR induced output ripple from D-CAP2™ mode control.
PWM Frequency and Adaptive On-Time Control
TPS56528 uses an adaptive on-time control scheme and does not have a dedicated on board oscillator. The
TPS56528 runs with a pseudo-constant frequency of 650 kHz by using the input voltage and output voltage to
set the on-time one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the
output voltage; therefore, when the duty ratio is VOUT/VIN, the frequency is constant.
Advanced Auto-Skip Eco-Mode™ Control
The TPS56528 is designed with advanced auto-skip Eco-mode™ to increase higher light load efficiency. As the
output current decreases from heavy load condition, the inductor current is also reduced. If the output current is
reduced enough, the inductor current ripple valley reaches the zero level, which is the boundary between
continuous conduction and discontinuous conduction modes. The rectifying low-side MOSFET is turned off when
its zero inductor current is detected. As the load current further decreases the converter run into discontinuous
conduction mode. The on-time is kept approximately the same as is in continuous conduction mode. The off-time
increases as it takes more time to discharge the output capacitor to the level of the reference voltage with
smaller load current. The transition point to the light load operation IOUT(LL) current can be calculated in
Equation 1
V
(
-VOUT ×V
)
1
IN
OUT
IOUT (LL)
=
×
2× L× fsw
VIN
(1)
Soft Start and Pre-Biased Soft Start
The TPS56528 has an internal 1.0ms soft-start. When the EN pin becomes high, internal soft-start function
begins ramping up the reference voltage to the PWM comparator.
The TPS56528 contains a unique circuit to prevent current from being pulled from the output during startup if the
output is pre-biased. When the soft-start commands a voltage higher than the pre-bias level (internal soft start
becomes greater than feedback voltage VFB), the controller slowly activates synchronous rectification by starting
the first low side FET gate driver pulses with a narrow on-time. It then increments that on-time on a cycle-by-
cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle of the converter. This
scheme prevents the initial sinking of the pre-bias output, and ensure that the out voltage (VO) starts and ramps
up smoothly into regulation and the control loop is given time to transition from pre-biased start-up to normal
mode operation.
Copyright © 2013, Texas Instruments Incorporated
7
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