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TPS51225B Datasheet(PDF) 10 Page - Texas Instruments

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No. de pieza TPS51225B
Descripción Electrónicos  Dual Synchronous, Step-Down Controller with 5-V and 3.3-V LDOs
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Fabricante Electrónico  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
Logo TI1 - Texas Instruments

TPS51225B Datasheet(HTML) 10 Page - Texas Instruments

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IN
OUT
OUT
OUT LL
SW
IN
V
V
V
1
I
2 L f
V
TPS51225, TPS51225B, TPS51225C
SLUSAV0A – JANUARY 2012 – REVISED JUNE 2012
www.ti.com
DETAILED DESCRIPTION
PWM Operations
The main control loop of the switch mode power supply (SMPS) is designed as an adaptive on-time pulse width
modulation (PWM) controller. It supports a proprietary D-CAP™ mode. D-CAP™ mode does not require external
conpensation circuit and is suitable for low external component count configuration when used with appropriate
amount of ESR at the output capacitor(s).
At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or enters the ON state. This
MOSFET is turned off, or enters the ‘OFF state, after the internal, one-shot timer expires. The MOSFET is turned
on again when the feedback point voltage, VVFB, decreased to match the internal 2-V reference. The inductor
current information is also monitored and should be below the overcurrent threshold to initiate this new cycle. By
repeating the operation in this manner, the controller regulates the output voltage. The synchronous low-side
(rectifying) MOSFET is turned on at the beginning of each OFF state to maintain a minimum of conduction loss.
The low-side MOSFET is turned off before the high-side MOSFET turns on at next switching cycle or when
inductor current information detects zero level. This enables seamless transition to the reduced frequency
operation during light-load conditions so that high efficiency is maintained over a broad range of load current.
Adaptive On-Time/ PWM Frequency Control
Bacause the TPS51225/B/C does not have a dedicated oscillator for control loop on board, switching cycle is
controlled by the adaptive on-time circuit. The on-time is controlled to meet the target switching frequency by
feed-forwarding the input and output voltage into the on-time one-shot timer. The target switching frequency is
varied according to the input voltage to achieve higher duty operation for lower input voltage application. The
switching frequency of CH1 (5-V output) is 300 kHz during continuous conduction mode (CCM) operation when
VIN = 20 V. The CH2 (3.3-V output) is 355 kHz during CCM when VIN = 20 V.
Light Load Condition in Auto-Skip Operation (TPS51225/C)
The TPS51225/C automatically reduces switching frequency during light-load conditions to maintain high
efficiency. This reduction of frequency is achieved smoothly and without an increase in output voltage ripple. A
more detailed description of this operation is as follows. As the output current decreases from heavy-load
condition, the inductor current is also reduced and eventually approaches valley zero current, which is the
boundary between continuous conduction mode and discontinuous conduction mode. The rectifying MOSFET is
turned off when this zero inductor current is detected. As the load current further decreases, the converter runs in
discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that
requires the next ON cycle. The ON time is maintained the same as that in the heavy-load condition. In reverse,
when the output current increase from light load to heavy load, the switching frequency increases to the preset
value as the inductor current reaches to the continuous conduction. The transition load point to the light load
operation IOUT(LL) (i.e. the threshold between continuous and discontinuous conduction mode) can be calculated
as shown in Equation 1.
where
fSW is the PWM switching frequency
(1)
Switching frequency versus output current during light-load conditions is a function of inductance (L), input
voltage (VIN) and output voltage (VOUT), but it decreases almost proportional to the output current from the
IOUT(LL).
10
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Product Folder Link(s) :TPS51225 TPS51225B TPS51225C


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