HISAT

DC/DC Converter

Next Generation Hi-SAT COTTM DC/DC Converter

Advanced Control loop adjusts the on-time based on input and output voltages resulting in an improved fixed frequency operation and predictable EMI.

TOREX Semiconductor’s newest Hi-SAT COTTM Step Down DC/DC Converters are the XC9266, XC9273/74/75 and XC9281/82 Synchronous Buck Regulators. This series of products build on the success of the 1st Generation Hi-SAT COTTM family.

This new product family incorporate an Improved Constant On-Time Architecture and a Higher Accuracy Voltage Reference Circuit which are required to regulate “Sub 1v Core Voltages” that are required for many applications in the electronic market place today.

Typical applications would include network and communication equipment, power supply modules and other embedded products which can utilize the fast-transient response characteristics of the Hi-SAT COTTM architecture. It is aimed at ASIC core voltage regulators, DSP and FPGA core power supplies.

This new range of DC/DC Convertors exhibit advantages over traditional COT Architectures and indeed our 1st Generation of Hi-SAT COTTM products, as shown in the table below:

 

Characteristics Comparison Between Topologies

FUNCTION COT HI-SAT COT (1st Gen) HI-SAT COT (2nd Gen)
Line Regulation Excellent Excellent Excellent
Load Regulation Excellent Excellent Excellent
Stable Operation with Wide
Range of Load Capacitors
NO YES YES
Minimum Output Voltage < 1V >1V <1V
Switching Frequency Variable Pseudo-Fixed Fixed
Transient Response Ultra-Fast Ultra-Fast Ultra-Fast
Accuracy Good Good Excellent
Power Save Mode Inherent Inherent Inherent
Output Capacitance Limited Flexible Flexible

The Hi-SAT Constant on-time topology in common with hysteretic based topologies, enables a fast-load transient response. This results in simple, space and cost-efficient power management solutions. However, compared with conventional constant on-time and hysteretic topologies, the new design techniques implemented provides for a much more constant switching frequency.

 

The block diagram below shows the simplified schematic of the XC9273 a 3A Step Down DC/DC Convertor Product which utilizes the Hi-SAT COT architecture.

 

 

The introduction of the High Gain Error Amp shown above means that very small deviations in FB voltage are sensed and passed to the input of the PWM comparator this is required to maintain accurate regulation for DC/DC Convertors with low output voltages.

 

DC/DC Theory Of Operation

The On Time (ton) is determined by the input voltage and output voltage, and turns on the Driver & High side FET for a fixed time.  During the off time (toff), the FB voltage is compared to a reference voltage by the Error Amp, the Error Amp output is sent to the PWM comparator.  The PWM comparator compares this Error Amp output signal to a reference voltage, and if the signal is lower than the reference voltage, then the SR latch is set.  This results in the On-Time Generator restarting.  By doing this PWM operation takes place with the off time controlled to the optimum duty cycle ratio and the output voltage is stabilized.

 

The phase compensation circuit optimizes the frequency characteristics of the error amp, and generates a ramp wave which is like the ripple voltage that occurs on the output to modulate the output signal of the error amp.  This enables a stable feedback system to be obtained even when a low ESR capacitor such as a ceramic capacitor is used.

 

This type of architecture results in an ultra-fast control loop to minimize undershoots and overshoots during high speed load current transients, this can minimize the risk of a system reset caused by the power supplying drooping too low or indeed causing damage to the downstream MPU when an overshoot occurs when the load is removed.

 

Faster Transient Response

As discussed above, Hi-SAT COTTM offers a big improvement in transient response.  As can be seen in the graphs below there is a ~ x6 improvement when the high load current is applied and ~ x9 improvement when the high load condition is removed. It can also be observed the loop response time is much faster resulting in the DC output settling to its regulated steady state value ~ x10 faster.

 

 

Higher Output Voltage Accuracy

A further improvement in the 2nd Generation Architecture is observed in the Accuracy of the DC Output Voltage as shown in the graphs below. This is achieved by a far superior Temperature Compensated Voltage Reference Circuit which can achieve +/- 1% FB voltage Accuracy over the temperature range -20°C to 105°C.

 

 

There is a real need to maintain a highly accurate DC Output Voltage in Low Output Voltage DC/DC Convertors which supply power to low voltage MPU, as many of these types of next generation MPU require very strict input voltage tolerances to optimize the performance.

 

More Stable Switching Frequency

A final improvement in the 2nd Generation Hi-SAT COT series is the improvement in the Switching Frequency Accuracy over load current and input voltage which is an inherent issue with traditional COT architectures. This makes it easier for system designers to filter the noise generated by the DC/DC Convertor, resulting in less interference to other circuits. The graphs below shows the improvements made between the 1st and 2nd Generation Hi-SAT COT devices.

 

 

New and improved design techniques have made the Switching frequency almost constant over load current and line voltage specifications compared to the XC9257 (a first generation Hi-SAT COT device).

 

Smaller PCB Area

The advances outlined previously have allowed the system solutions to become smaller due to the more accurate higher switching frequency meaning smaller inductors can be used and an ultra-fast control loop means smaller output capacitors are required whilst maintaining very little voltage drop on the output.

XC9266  3MHz, 6A DC/DC

L = 0.22μH, CIN = COUT = 47 μF

EVB is only 45mm2.
~35% smaller than COT DC/DC solution

 

With the EVB shown above the VOUT recovers in 3.5μs with only 80mV drop when transitioning from 100mA to 6A (VIN=5.0V, VOUT=1.8V)

 

Summary

Designers wishing to implement point-of-load power circuits now have an improved family of parts from Torex to choose from. These parts efficiently manage current in standby and full current modes, using an advanced on-time topology to provide fast-transient response, reduced output capacitance, and constant switching frequencies.

 

Torex 2nd Generation Hi-SAT COT Family of DC/DC Converters

SERIES VIN RANGE VOUT RANGE IOUT FSW PACKAGE
XC9266 2.7V ~ 5.5V 0.8V ~ 3.6V 6.0A 1.2MHz / 3MHz QFN-24
XC9273 2.7V ~ 5.5V 0.8V ~ 3.6V 3.0A 1.2MHz / 3MHz QFN-24
XC9274/75 2.7V ~ 5.5V 0.8V ~ 3.6V 3.0A 1.2MHz / 3MHz SOP-8FD
XC9281/82 2.7V ~ 5.5V 0.8V ~ 3.6V 0.6A 6MHz LGA-6B01
WLP-5-06