Datasheet

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DETAILED DESCRIPTION
PWM Operation
Current Mode
D-CAP™ Mode
TPS51120
SLUS670B – JULY 2005 – REVISED FEBRUARY 2007
The switching mode power supply (SMPS) block of TPS51120 supports an adaptive on time control
pulse-width-modulation (PWM). Switching frequency is selectable from four choices for maximum efficiency
(5 V/180 kHz, 3.3 V/270 kHz), minimum component size (5 V/380 kHz, 3.3 V/580 kHz) or the other two
intermediates. The TPS51120 supports both true current mode control and D-CAP™ mode control, selectable
up to the requirements from system design. All N-channel MOSFET totem-pole architecture is employed for
external switches. The synchronous top (high-side) MOSFET is turned on, or is “SET”, at the beginning of each
cycle. This MOSFET is turned off, or is “RESET” after a constant “on-time” period which is defined by the
frequency of customer’s choice and input and output voltage ratio. The top MOSFET is turned on again if
inductor current is reduced to meet both conditions of,
1. the current level corresponds to the error amount of output voltage and,
2. below the overcurrent limit level
Repeating operation in this manner, the controller regulates the output voltage. The synchronous bottom
(low-side) or the rectifying MOSFET is turned on each cycle in the negative phase to the top MOSFET to keep
the conduction loss minimum. The rectifying MOSFET turns off on the event reverse inductor current flow is
detected. This enables seamless transition to skip mode function so that high efficiency is kept over a broad
range of load current. At the beginning of the soft start period, the rectifying MOSFET remains in the off state
until the top MOSFET is turned on for at least once.
The current mode scheme is a sequence of feedback control described as follows. The output voltage is
monitored at the middle point of voltage divider resistors and fed back to a transconductance amplifier. The
amplifier outputs target current level proportional to error amount between the feedback voltage and the internal
1 V reference voltage. The inductor current level is monitored during the off-cycle, when rectifying MOSFET is
turned on. The PWM comparator compares the inductor current signal with this target current level that is
indicated at the COMP pin voltage. When both signals are equal (at the valley of the current sense signal), the
comparator provides the “SET” signal to the gate driver latch. The current mode option has relatively higher
flexibility by the external compensation network provided to the COMP pin. And it is suitable for lowest ripple
design with output capacitor(s) having ultra-low ESR. More detail information about loop compensation and
parameter design can be found in the Loop Compensation and External Parts section. When sensing the
inductor current, accuracy and cost always trades off. In order to give the circuit designer a choice between
these two, TPS51120 supports both of external resistor sensing and MOSFET R
DS(on)
sensing. Please contact
factory for current mode EVM with R
SENSE
capability.
The D-CAP™ mode operation is enabled by tying the COMP pin to V5FILT. In this mode, the PWM comparator
monitors the feedback voltage directly and compares the voltage with the internal 1-V reference. When both
signals are equal at the valley of the voltage sense signal, the comparator provides the “SET” signal to the top
MOSFET gate driver. Because the compensation network is implemented on the part and the output waveform
itself is used as the error signal, external circuit design is largely simplified. Another advantage of the D-CAP™
mode is its inherent fast transient response. A trade-off is a sufficient amount of ESR required in the output
capacitor. SPCAP or POSCAP is recommended. The inductor current information is still used in the D-CAP™
mode for over current protection and light load operation. Do NOT neglect current sensing design in this mode.
To summarize, the D-CAP™ mode is suitable for the lowest external component count with the fastest transient
response, but with relatively large ripple voltage. It is easy to design the loop once appropriate output capacitor
and inductor current ripple is selected. Please refer to loop compensation and parameter design in the Loop
Compensation and External Parts section for more information.
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