SLUS935A – MAY 2009 – REVISED JUNE 2009 ............................................................................................................................................................... www.ti.com
The TPS51218 is a high-efficiency, single channel, synchronous buck regulator controller suitable for low output
voltage point-of-load applications in notebook computers and similar digital consumer applications. The device
features proprietary D-CAP™ mode control combined with adaptive on-time architecture. This combination is
ideal for building modern low duty ratio, ultra-fast load step response DC-DC converters. The output voltage
ranges from 0.7 V to 2.6 V. The conversion input voltage range is from 3 V to 28 V. The D-CAP™ mode uses the
ESR of the output capacitor(s) to sense current information. An advantage of this control scheme is that it does
not require an external phase compensation network, helping the designer with ease-of-use and realizing low
external component count configuration. The switching frequency is selectable from four preset values using a
resistor connected from the RF pin to ground. Adaptive on-time control tracks the preset switching frequency
over a wide range of input and output voltages, while it increases the switching frequency at step-up of load.
The RF pin also serves in selecting between auto-skip mode and forced continuous conduction mode for light
load conditions. The strong gate drivers of the TPS51218 allow low RDS(on) FETs for high current applications.
When the EN pin voltage rises above the enable threshold, (typically 1.2 V) the controller enters its start-up
sequence. The first 250
µs calibrates the switching frequency setting resistance attached at RF to GND and
stores the switching frequency code in internal registers. A voltage of 0.1 V is applied to RF for measurement.
Switching is inhibited during this phase. In the second phase, internal DAC starts ramping up the reference
voltage from 0 V to 0.7 V. This ramping time is 750
µs. Smooth and constant ramp up of the output voltage is
maintained during start up regardless of load current. Connect a 1-k
Ω resistor in series with the EN pin to provide
TPS51218 does not have a dedicated oscillator that determines switching frequency. However, the device runs
with pseudo-constant frequency by feed-forwarding the input and output voltages into its on-time one-shot timer.
The adaptive on-time control adjusts the on-time to be inversely proportional to the input voltage and proportional
to the output voltage (tON ∝ VOUT / VIN ). This makes the switching frequency fairly constant in steady state
conditions over wide input voltage range. The switching frequency is selectable from four preset values by a
resistor connected to RF as shown in Table 1. (Leaving the resistance open sets the switching frequency to the
lowest value, 290 kHz. However, it is recommended to apply one of the resistances on the table in any
Table 1. Resistor and Switching Frequency
The off-time is modulated by a PWM comparator. The VFB node voltage (the mid point of resistor divider) is
compared to the internal 0.7-V reference voltage added with a ramp signal. When both signals match, the PWM
comparator asserts the set signal to terminate the off-time (turn off the low-side MOSFET and turn on high-side
MOSFET). The set signal becomes valid if the inductor current level is below OCP threshold, otherwise the
off-time is extended until the current level to become below the threshold.