SLYT846 February 2024 TPS62870 , TPS62870-Q1 , TPS62871 , TPS62871-Q1 , TPS62872 , TPS62872-Q1 , TPS62873 , TPS62873-Q1 , TPS62874-Q1 , TPS62875-Q1 , TPS62876-Q1 , TPS62877-Q1 , TPS6287B10 , TPS6287B15 , TPS6287B20 , TPS6287B25 , TPSM8287A06 , TPSM8287A10 , TPSM8287A12 , TPSM8287A15
A common drawback with constant on-time (COT) control topologies is the switching frequency variation and inability to synchronize to an external clock. TI’s fixed-frequency direct control with seamless transition into power save mode (fixed-frequency DCS-Control) topology builds on the popular COT DCS-Control topology with its fast transient response, and adds an oscillator to achieve fixed-frequency operation with optional clock synchronization. This combination enables applications that require both a fast transient response and have specific noise or frequency requirements.
Other features such as differential remote sensing, external control-loop compensation and stackability support the demanding transient requirements of higher-current processors found in noise-sensitive applications, including automotive infotainment and advanced driver assistance systems (ADAS), communications equipment optical modules, industrial test and measurement, medical, and aerospace and defense. This article provides an overview of the fixed-frequency DCS-Control topology, discussing its excellent transient response, constant and synchronize-able switching frequency, lower-ripple power-save mode, and stackability for higher currents.
Figure 1 shows the basic block diagram of the DCS-Control topology [1]. Both the output-voltage sense (VOS) and feedback (FB) pins provide the inputs to the control loop for proper regulation. The VOS pin provides the topology’s fast transient response by directly feeding the output voltage into a ramp and then into the comparator, where it immediately affects the operating point. The FB pin is a lower-bandwidth path that provides highly accurate DC setpoint regulation. When combined in DCS-Control, the VOS pin’s AC path and FB pin’s DC path provide an accurate output voltage that also responds quickly to load transients.
A COT topology such as DCS-Control sets the on-time with a timer. By adjusting this on-time with the input and output voltage, the timer gives a reasonably constant frequency operation for most duty cycles in pulse-width modulation (PWM) mode. Equation 1 shows an example, where 416ns is the period for a 2.4MHz switching frequency:
However, the switching frequency is not precise enough for applications that require operation inside or outside of a specific frequency band. These applications generally require setting the switching frequency with an oscillator, such as in voltage- or current-mode control, and in some cases, the ability to synchronize with a system clock signal. Reference [2] offers further examples of the frequency variation of DCS-Control.
Figure 2 shows a basic block diagram of the fixed-frequency DCS-Control topology, as implemented in the 15A TPS62873 buck converter. The addition of an oscillator enables the direct setting of the switching frequency (fSW) in the same way as voltage- or current-mode control. Having an oscillator input into the control loop also provides the ability to synchronize the switching frequency to an applied clock signal.
Fixed-frequency DCS-Control, usually used in higher-current devices, uses differential remote sensing. The device regulates the voltage between the VOSNS and GOSNS pins, which are routed across the printed circuit board (PCB) to sense the output voltage directly at the load. Sensing at the load overcomes and compensates for not only the DC voltage drops across the PCB planes and traces, but also the delays that come from inductance between the device and the load. Both of these characteristics are important for maintaining very tight regulation across the load range and during load transients.
The differential remote-sensing signals are fed into the transconductance amplifier (gm), which compares their difference against the output voltage setpoint. (For simplicity, Figure 2 shows this setpoint as a voltage source in series with the GOSNS signal.) The COMP pin gives the output of this amplifier, which is compensated with a Type II (one pole, one zero) network to ground.
This external compensation allows you to optimize the control loop to any system need – from systems with strong load transients with large output capacitance, all the way down to systems with small or no load transients with very little output capacitance and small size. Unlike DCS-Control, the fast feedback path goes through this amplifier – not immediately to the comparator – where compensation component selection can increase (or decrease) the gain. If you need a stronger transient response, you increase the gain and add more output capacitance. If no strong transients are present in the application, you decrease the gain and use a minimal amount of output capacitance in order to achieve the smallest size.
The ability to adjust the transient response to the application needs enables tighter regulation under harsher transients than what is possible with the previous DCS-Control topology, and meets the requirements of demanding processor cores such as TI’s Jacinto™ J7 and MobileEye’s EyeQ6 [3-4]. Figure 3 shows a stack of three TPS62876-Q1 buck converters delivering a 46A load transient, while maintaining the output voltage within ±2% of the 0.875V setpoint.
A hysteretic comparator compares the COMP pin output and a replica of the inductor current, created by the τaux components, with slope compensation added to prevent subharmonic oscillations. The comparator’s output drives the Set-Reset (SR) latch circuitry, along with the clock, which controls the gate drivers and device operation. The oscillator controls the switching to occur exactly at the switching frequency.
The Set-Reset latch is a simplified representation of the detailed operation of the control block and is implemented to maintain the fast, hysteretic nature of DCS-Control and thus enable an immediate response to load transients. For example, during a load-dump transient (where the output voltage rises), the output of the hysteretic comparator has priority over the clock signal. The converter extends the off-time of the high-side MOSFET as needed to bring the output voltage back down with minimum overshoot. This is inherently improved behavior compared to textbook peak current-mode control, which switches at every clock cycle, continuing to add energy to the output, even while it is too high. By reducing the output-voltage overshoot, the converter significantly reduces the output capacitance, which is a key influence on the cost and size of the power supply.
In addition to maintaining the fast transient response, which can be further improved and tuned through the external compensation on the COMP pin, fixed-frequency DCS-Control provides a fixed switching frequency with a tight tolerance specification. Because the switching frequency is directly set with an oscillator instead of indirectly controlled with an on-timer, the frequency’s tolerance is specified in the device-specific data sheet. Table 1 and Table 2 compare the switching frequency specifications of the TPS62876-Q1, using the fixed-frequency DCS-Control topology, versus the typical frequency specification of the DCS-Control TPS62869 step-down converter.
Parameter | Test Conditions | MIN | TYP | MAX | Unit | |
---|---|---|---|---|---|---|
fSW | Switching Frequency | fSW = 1.5MHz, PWM operation | 1.35 | 1.5 | 1.65 | MHz |
fSW = 2.25MHz, PWM operation | 2.025 | 2.25 | 2.475 | |||
fSW = 2.5MHz, PWM operation | 2.25 | 2.5 | 2.75 | |||
fSW = 3MHz, PWM operation | 2.7 | 3 | 3.3 |
Parameter | Test Conditions | MIN | TYP | MAX | Unit | |
---|---|---|---|---|---|---|
fSW | PWM switching frequency | IOUT = 1A, VOUT = 0.9V | 2.4 | MHz |
Figure 4 and Figure 5 compare the actual variation of the switching frequency versus load current in an application. Both devices support power-save mode, which reduces the frequency at lower load currents (toward the left of both graphs). Operation in PWM mode (at higher currents) results in a precisely controlled switching frequency for fixed-frequency DCS-Control, while the switching frequency of DCS-Control increases slightly with an increasing load. In forced PWM mode (not shown), fixed-frequency DCS-Control maintains its constant frequency down to no load.
Besides power-save mode, there are two conditions where the switching frequency can deviate from the frequency set by the oscillator: during a strong load transient and if the minimum on-time is reached. When applying a heavy load, the high-side MOSFET may be on for longer than a full switching period, and when removing a heavy load, it may be off for longer than a full switching period. Both scenarios result in one or more pulses that are not present because of the extended on- or off-times.
If the minimum on-time of the high-side MOSFET is reached, both fixed-frequency DCS-Control and DCS-Control reduce the switching frequency in order to meet the minimum on-time and maintain output-voltage regulation. This is improved performance compared to some current-mode devices that maintain the frequency but let the output voltage rise in order to meet the required minimum on-time. While both fixed-frequency DCS-Control and DCS-Control reduce the switching frequency in the same way [2], fixed-frequency DCS-Control has fewer operating conditions during which the minimum on-time is reached, and the frequency reduced, because of its lower minimum on-time. For example, the TPS62876-Q1 specifies the 44ns maximum value of its minimum on-time at a 5V input voltage and across the operating temperature. Such a low value of minimum on-time enables lower output-voltage applications in automotive and aerospace and defense, for example, to operate in the higher-frequency region sometimes required by the overall system.