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FAQ: DC/DC Converters (Switching Regulators)


Q. What is the suitable inductor for each DC/DC converter (switching regulator)?

How to use / DC/DC: 005

A.

Each IC is designed for some specific inductance value shown in each datasheet. Use an inductor whose inductance is within the recommended range. Switching current is not flat. Select an inductor whose rated current is higher than the peak current of the switching.

The calculation method of the maximum output current flowing to the inductor is explained in datasheets of each product.

There are two types of rated current for inductors: One is the DC current based on decreasing inductance value by 30%, and the other is based on temperature-rise by 40%. Consider the smaller value of the two ratings as the rated current.


Q. What is the difference between DC/DC converters (switching regulators) and LDO regulators (linear regulators)?

Function, Term/ LDO: 009 | DC/DC: 001

A.

Both of DC/DC converters and LDO regulators convert an input direct current into a different direct current. The way of converting direct currents differs from each other. DC/DC converters regulate electric power by turning on and off switching elements (FETs, etc.). On the other hand, LDO regulators regulate power supply by controlling on-resistance of FETs.

DC/DC converters are highly efficient in converting electricity by the switching control. Depending on conditions such as output current, the efficiency can reach about 95%. Instead, there are some disadvantages. First, output voltages from this type of regulators contain ripples and switching noise, not suitable for noise-sensitive devices. Second, due to their complex control method, many external components are necessary compared with LDO regulators. Designing PCBs using DC/DC converters tends to be more difficult and more cost-consuming than those using LDO regulators.

The control method of LDO regulators is simple, so designing PCBs is easy. However, the conversion efficiency is inferior to that of DC/DC converters. In order to convert 5 V into 3 V, the efficiency by LDO regulators is 60%, while DC/DC converters can achieve 90% or more. Besides, if dropout voltage and output current are large, heat dissipation must be considered.

  DC/DC converters (switching regulators) LDO regulators (linear regulators)
Efficiency High Low
Price High Low
Noise Loud Low
Design Difficult Easy
Components Many Few
Load Current Large Small

Q. What is PWM/VFM?

Function, Term/ DC/DC: 002

A.

Both of PWM and VFM represent the control methods of DC/DC converters (switching regulators).

VFM (Variable Frequency Modulation)

VFM is also called PFM (Pulse Frequency Modulation). In VFM, the output voltage is regulated by controlling oscillator frequency (fosc). In the case of Ricoh DC/DC converters, ON time (Ton) of switching is fixed and switching pulses are reduced according to the load current.

Ton = Duty / fosc

In VFM operation, there is no continuous mode, because the duty cycle is constant regardless of input/output conditions.

Advantages

  • Efficiency of VFM mode is better than PWM mode because the oscillator frequency changes lower at light load.
  • Phase margin of VFM is larger than that of PWM.

Fig. 1 Comparison of efficiency between PWM and VFM

Fig. 1 Comparison of efficiency between PWM and VFM

Fig. 2 Lx waveform VFM (Iout = 2 mA)

Fig. 2 Lx waveform VFM (Iout = 2 mA)

Disadvantages

  • Noise suppression of VFM is more difficult than that of PWM because fosc is inconsistent, which affects noise level. (Note: abnormal noise can be generated by the inductor, when the fosc reaches the audible band.)
  • The output voltage falls at heavy load because VFM operates in discontinuous mode.
  • Ripples in VFM are bigger than those in PWM, because of the constant Ton. Under heavy-load conditions, one-time switching pulse cannot provide sufficient energy. As a result, the number of pulses during Ton increases, making ripples larger.

Lx waveform and ripple waveform at VFM mode

Fig. 3 Lx waveform and ripple waveform at VFM mode

PWM (Pulse Width Modulation)

The fosc is constant and the Ton (Duty ratio) changes according to the amount of output load.

Advantages

  • Noise suppression of PWM is relatively easier than that of VFM because fosc is constant.
  • Ripples are suppressed because Ton changes according to the amount of output load.
  • PWM mode DC/DC converters can operate at heavy load due to continuous mode.

Disadvantages

  • The efficiency of PWM is not better at light load than VFM because the fosc is constant regardless of the amount of output load.
  • Phase margin should be cared. (Note: Use the equivalent external components shown in datasheets.)

Fig. 4 Images of waveform

Fig. 4 Images of waveform

Fig. 5 Transition image of ripple voltage

Fig. 5 Transition image of ripple voltage

Ripples get larger as the output load increases due to prolonged Ton. At continuous operation, ripples become almost stable due to constant Ton.


Q. What is the function of the current limit circuit of DC/DC converters?

Function, Term/ DC/DC: 003

A.

The current limit circuit prevents the inductor current from being beyond the limit value. The current limit circuit limits the inductor current in every switching cycle.
Depending on the product, types of protection operation are different. See datasheets for details.


Q. What is the function of the BST pin?

Function, Term/ DC/DC: 004

A.

The BST pin is necessary for DC/DC converters with a bootstrap circuit.

A bootstrap circuit generates the gate drive voltage for a high-side N-channel driver by constructing a boost charge pump circuit utilizing switching operations of a DC/DC converter.*

  • *An N-channel driver has higher driving capability than a P-channel driver. Therefore, the driver size can be small compared with a P-channel driver.

By connecting a capacitor (CBST) between the BST and Lx pins, the voltage between the two pins is regulated to 5 V typically.

When the Lx voltage is low, the CBST is charged. Then, when the BST voltage exceeds the Lx voltage by 5 V, the N-channel driver can be driven.
Any types of capacitors are available as CBST, if the capacitance is equal or more than the recommended value.

Gate Voltage over VIN is necessary

CBST is charged when LX is Low

Ricoh DC/DC converters (switching regulators) achieve low supply current, high efficiency and miniaturization by CMOS analog technology and by elaborate design skills. Our products can meet various needs of customers.

In order to correspond to the different customer’s requirements, we have a wide lineup of DC/DC converters featuring operating input voltages from 0.6 V to 34 V, output currents from 1 mA to 14 A, and low supply currents from 0.3 µA.

Our converters include the following protections: three types of overcurrent protection (foldback, latch, or reset), overvoltage protection (OVP), undervoltage lockout (UVLO), and thermal shutdown (TSD).

Various kinds of DC/DC converters are available: buck, boost, buck-boost, and multiple converters in a single package. Moreover, three product grades are offered: consumer, industrial, and automotive. We will provide the most appropriate product for your applications.