This article reviews some common feedback control architectures of switch-mode power supplies. The advantages and disadvantages of each architecture are explained and Torex’s HiSAT-COT™ control architecture is discussed in detail. The HiSAT-COT is designed for applications that require ultra-fast transient response and operation at a fixed frequency. These applications include network and communication equipment, power supply modules and other embedded products.
Switch-mode power converters are commonly used in portable systems to maximize battery life. They can be used to efficiently step-down (buck) or step-up (boost) the battery voltage to higher levels. Switch-mode power supplies are available with voltage mode, current mode and constant on time feedback control architectures (Figure 1). Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are among the operation modes to control the output voltage. PWM provides regulation by adjusting the on/off time ratio at a constant switching frequency, whereas PFM uses a fixed on/off time ratio and a variable frequency.
Figure 1: PWM DC supplies can be either voltage mode or current mode while PFM DC supplies can be constant-on-time devices. (Image source: Torex)
What is the difference between PWM and PFM?
A PWM converter is a DC/DC power converter architecture that uses a fixed-frequency oscillator to drive the power switches and transfer energy from the input to the output. The drive signal used is constant in frequency but varies in its duty cycle (ratio of power-FET on time to the total switching period). The clock frequency is fixed and the duty cycle is adjusted based on operating conditions.
An architecture that uses a variable frequency to drive power switches in DC-DC power-converters is called PFM, or “pulse-frequency modulation”. The drive signal’s frequency is directly controlled to regulate the output voltage. DC/DC converters with constant-on-time or constant-off-time control are typical examples of the PFM architecture.
Voltage mode control
Figure 2 shows a step-down power converter with voltage mode control. This architecture uses a single voltage feedback path. The error voltage is compared to ramp by a PWM comparator which drives the control block to generate a PWM signal to control the high side switch.
Figure 2: Step-down power converter with voltage mode control. (Image source: Torex)
The advantages of voltage mode control include:
- Less sensitive to noise than comparable current mode control
- Singe feedback loop makes the analysis easier
- Can operate over a wide range of input voltages and duty cycles
The disadvantages of voltage mode control include:
- Loop gain is proportional to VIN
- Requires complex compensation
- Slow response – changes in the input voltage are sensed at the output
- Current limiting must be done separately
Figure 3 shows a step-down power converter with current mode control. This architecture uses two feedback paths to sense the output voltage and inductor current.
Figure 3: Step-down power converter with current mode control. (Image source: Torex)
The advantages of current mode control include:
- Fast response to line and load changes
- Easier to compensate
- Current limiting
- Simplified load sharing
The disadvantages of current mode control include:
- More difficult circuit analysis since there are two loops
- Resonances in the power stage can introduce noise into the inner control loop1
- Need for slope compensation
Figure 4 shows a step-down power converter with constant-on-time (COT) control. In the COT control architecture there is no clock and the frequency can vary.
Figure 4: Step-down power converter with constant-on-time control. (Image source: Torex)
The advantages of COT mode control include:
- Minimal number of external components required
- Fast transient response
- No compensation needed
- Good efficiency over a wide range of load conditions
The disadvantages of current mode control include:
- Frequency variations
- Requires output ripple suppression
- Sensitive to output noise
- Needs overcurrent protection
The biggest drawback of the COT architecture is variation of frequency that can cause electromagnetic interference (EMI) in sensitive circuits adjacent to the regulator. Torex addresses this weakness with its proprietary HiSAT-COT Control architecture. HiSAT-COT stands for High Speed Circuit Architecture for Constant On Time. Figure 5 shows the comparison between COT and Torex’s HiSAT-COT Control architecture.
Figure 5: Comparison of Torex’s HiSAT COT devices with normal COT converters. (Image source: Torex)
Torex’s 2nd Generation HiSAT-COT control architecture provides fixed frequency operation and improves output accuracy. Figure 6 shows a typical HiSAT-COT frequency variation vs. load current curve.
Figure 6: Frequency variation vs. load current for Torex’s first and second generation Hi-SAT COT devices. (Image source: Torex)
Compared with conventional DC-DC products on the market, the HiSAT-COT control produces an ultra-fast transient response, as shown in Figure 7, and requires no external compensation. There is an approximate 6x improvement when the load is applied and an approximate 9x improvement when the load condition is removed.
Figure 7: Load transient response comparison between a Torex second generation HiSAT-COT and a standard PWM controlled converter. (Image source: Torex)
The 2nd generation HiSAT-COT control architecture also improves the accuracy of the DC output voltage. This is achieved by a superior temperature compensated voltage reference circuit which can achieve +/-1% FB (frequency bandwidth) voltage accuracy over temperature (Figure 8). There is a need to maintain high voltage accuracy at low voltages as many MPU (microprocessor unit) loads require tight input voltage tolerances.
Figure 8: Typical output voltage accuracy over frequency for Torex’s first and second generation HiSAT-COT devices. (Image source: Torex)
Torex has recently announced the XC9281/XC9282, a new series of HiSAT-COT control, extremely small 600 mA step-down DC-DC converters. The devices operate from input voltages of 2.5 V to 5.5 V, with the output voltage being adjustable from 0.7 V to 3.6 V. The device only consumes 11 µA of quiescent current. Operating at a 6 MHz switching frequency, a 0.47 µH inductor with a size of 1.0 x 0.5 mm can be used. A 0.6 x 0.3 mm ceramic capacitor can be used for the input capacitance (CIN) and the output capacitance (CL). Using these components results in the mounting area, including the peripheral components, of only 6.6 mm2. (Figure 9)
Figure 9: Mounting area of Torex’s XC9281/XC9282 devices including the peripheral components. (Image source: Torex)
Table 1 shows Torex’s 2nd generation of HiSAT-COT products.
Table 1: Torex’s second generation of HiSAT-COT products. (Image source: Torex)
Designers wishing to design point-of-load power circuits now have a family of Torex products to choose from. These devices improve efficiency over a wide operating condition. By operating at high switching frequency and providing ultra-fast transient response, the HiSAT-COT family of products reduces the total solution size by reducing the size of the inductor and output capacitor.
- “Switching Power Supply Topology Voltage Mode vs. Current Mode,” Robert Mammano, Unitrode, DN-62, June 1994.