How far have we come? Low-noise switching regulator "Silent Switcher 3"

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ProductsPickup Power SuppliesAnalog Devices

Newly released LT83401/2with 42Vin support

Switching regulators are noisy, so in circuits that are sensitive to noise, it is common to add an LDO after the regulator. However, adding an LDO reduces efficiency and increases the design burden. Have you ever thought, "If I had a low-noise switching regulator, I wouldn't need an LDO?"

In fact, Analog Devices offers an unprecedented ultra-low noise switching regulator, the Silent Switcher 3. This product is ideal for circuits that previously used LDOs as a noise countermeasure, or circuits that require low noise and high current.

Here we will introduce the features, benefits, and lineup of Silent Switcher 3.

Issues with switching power supplies

EMI noise is caused by sudden current changes in the hot loop and ringing during switching.

In general switching regulators, ringing occurs in the hot loop during switching operation, which causes noise. It is possible to use filters or mechanical shields to reduce noise, but this has the drawback of increasing costs and the board area.

Additionally, in noise-sensitive circuits, a common method of reducing noise is to add an LDO in the subsequent stage, but this brings issues such as heat generation from the IC, reduced efficiency, and an increased number of components.

Analog Devices' low-noise switching regulator, Silent Switcher 3, can solve these problems. It dramatically reduces low-frequency noise and can be used in noise-sensitive circuits without an LDO.

What is Silent Switcher?

"Silent Switcher" is Analog Devices' proprietary ultra-low noise DC-DC converter architecture that achieves "low EMI", "high efficiency", and "high switching frequency" without any trade-offs.

Silent Switcher Fundamentals

The first generation, Silent Switcher 1, features the following technologies:

・Bonding wire is changed to flip chip to suppress ringing

Bonding wires and copper pillars — The wiring of a typical IC is thin and long, which generates a parasitic L component and causes ringing.
Silent Switcher minimizes parasitic L components by making wiring thicker and shorter!

The bonding wires used in typical IC wiring are thin and long, which causes large parasitic inductance and ringing. Silent Switcher 1 does not use bonding wires and instead uses flip-chip connections. The thick and short copper pillars reduce parasitic inductance and suppress ringing.

・The hot loop was divided into two, successfully confining the magnetic field.

Splitting the hot loop into two smaller loops reduces harmonic energy, and confining the magnetic field between the two loops prevents EMI propagation. — There are two input terminals, one on the left and one on the right, and the green and blue hot loops generate a red magnetic field.
The magnetic field traps EMI noise.

The hot loop, which is the source of noise, is divided into two smaller loops and arranged to surround the IC, successfully containing the magnetic field and preventing the propagation of EMI.

・High efficiency even at high switching frequencies allows for smaller circuits

It features an internal switch driver that minimizes power loss during switching, resulting in fast and clean switching edges.

Silent Switcher complies with the most stringent noise standard, CISPR25 Class 5, and can solve the issues of noise, heat, efficiency, and size in power supply design all at once.

Silent Switcher 2

The second generation, Silent Switcher 2, has an input capacitor built into the package, eliminating the impact of PCB layout. Silent Switcher 1 required external capacitors to be placed symmetrically on both sides, but Silent Switcher 2 eliminates this constraint, allowing for a simpler, more space-saving design. In addition, the inclusion of an input capacitor makes the hot loop shorter, improving noise performance.

Silent Switcher 3

The third generation, Silent Switcher 3, achieves "reduction of low frequency noise (10Hz-100kHz)" and "ultra-fast transient response" in addition to the features of Silent Switcher 2.

Silent Switcher 3 uses the current reference technology of Analog Devices' LT3045, the market's leading ultra-low noise LDO. By changing the conventional bandgap reference, which has a high noise component, to a current reference, we have succeeded in significantly reducing low-frequency noise. This allows the switching regulator to achieve noise performance comparable to that of an LDO.

The current reference method used in the industry's No. 1 low noise LDO "LT3045" is now applied to Silent Switcher!

Silent Switcher 3 Features

Specifically, what kind of noise characteristics does Silent Switcher 3 have? The table below compares the low-frequency noise from 10 Hz to 100 kHz of the Silent Switcher 3"LT8625S" with other solutions. The noise of the LT8625S is almost equivalent to that of a Li-ion battery, and is overwhelmingly lower than that of conventional LDOs.

Integrated Noise from 10Hz-100kHz
LT3045 (Industry's lowest noise LDO)	0.8μV_RMS
Lithium-ion battery	2.7μV_RMS
LT8625S (high bandwidth)	2.7μV_RMS
LT8625S (low bandwidth)	3.7μV_RMS
General low noise LDO	Approximately 4μV _RMS
LT8643S (SS2 in unity gain fb)	123μV_RMS

DCDC with lower noise than LDO and equivalent to battery

Noise Density Comparison with LT8625S and LT3045 — The LT3045 is one of the lowest noise LDOs on the market

The figure on the right compares the noise spectral density of the LT8625S with that of the market-leading ultra-low noise LDO, the LT3045. Especially at low frequencies, the LT8625S has almost the same characteristics as the LT3045, and has superior noise performance.

・Can also be used for noise-sensitive RF power supplies

Silent Switcher 3 can also be used in noise-sensitive RF power supplies such as the ADF4372 PLL. — ADF4372: 16 GHz PLL frequency synthesizer.
Evaluation board using Silent Switcher 3 (LT8625SP) for 5V power supply and LT3045 for 3.3V power supply

The Silent Switcher 3 's excellent noise characteristics allow it to be used in noise-sensitive circuits where switching regulators have traditionally been difficult to use.

For example, it was previously difficult to directly power a PLL device from a switching regulator. However, with Silent Switcher 3, you can achieve noise performance almost equivalent to that of the ultra-low noise LDO"LT3045" simply by adding an external second-stage LC post-filter. This makes it possible to use it as a GHz-band power supply for PLL devices and other devices.

Noise waveform when ADF4372 is powered by LT3045 at both 5V and 3.3V

The waveforms created using the LT3045 for both 5V and 3.3V are in blue.
When using the LT8625SP, some frequencies (several tens of kilohertz to several hundreds of kilohertz) deteriorate slightly.

Adding an external LC post-filter to the 5V power supply of the ADF4372 significantly reduces noise.

By adding an external LC post filter, the performance is almost equivalent to LT3045 x 2. It can also be used as a GHz band power supply!
*The LT3045isADI'slowest noiseLDO.

・No need for LDO, saving space and reducing costs

Benefits of using Silent Switcher 3: Circuits that previously consisted of a DCDC → LDO can now be realized with just one DCDC

By using Silent Switcher 3, you can achieve a low-noise power supply without adding an LDO. For example, a circuit that was previously configured with two stages, such as "DC (12V) → DCDC (intermediate voltage 6V) → LDO (5V)", can be simply configured as "DC (12V) → Silent Switcher 3 (5V)".

Eliminating the need for an LDO reduces the number of components, simplifies the design, saves space, and reduces costs.

・Ideal for circuits that require low noise and high current

The Silent Switcher 3 is ideal for circuits requiring high current and low noise.

Due to the characteristics of LDOs, power loss is large at high currents, and heat generation becomes an issue. Conventionally, in high-current circuits that require low noise, it was necessary to use high-performance LDOs with good heat dissipation characteristics or to add an external heat sink, which increased costs and board area.

Silent Switcher 3 achieves both high efficiency and low noise, and suppresses temperature rise in ICs even at high currents. It also eliminates the need for an additional heat sink, contributing to reduced component count and space savings.

Silent Switcher 3 is ideal for applications requiring low noise but high current consumption!

If you try to design with an ambient temperature of 40℃ and Tj=100℃ or less, then θjA×2.04W+40℃=100℃

θjA＝(100-40)℃/2.04W=29.4℃/W

It is necessary to use an LDO that can handle large currents with good heat dissipation characteristics, or to implement a heat sink.

DCDC allows for high efficiency and current flow, and SilentSwitcher3 also produces low noise!

θjA=46℃/W

46℃/W×0.3W+40℃=53.8℃

There is a large margin for 100°C, so no heat sink is required!

- Supports fast transient response, settling to within 0.2% in less than 5μs

The Silent Switcher 3 supports ultra-fast transient response, making it suitable for powering FPGAs, RF transceivers, and more.

Recent FPGAs and RF transceivers have increasingly stringent power supply requirements, and if the transient response is insufficient, there is a risk that the voltage will deviate from the specified value, resulting in malfunction.

The Silent Switcher 3 can settle to within 0.2 % in less than 5 μs, making it ideal for devices requiring fast transient response.

Silent Switcher 3 Lineup

The Silent Switcher 3 portfolio includes:

model number	Vin (V)	Iout (A)	package
LT83401	42	1	3×2 LFCSP	Pin compatible
LT83402	42	2.5
LT83201	18	1
LT83203		3
LT83205		5
LT8622S		2	4×3 LQFN	Pin compatible
LT8624S		4
LT8625S/SP		8
LT8625SP-1		8	4×4 LQFN	Pin compatible
LT8627SP		16	4×4 LQFN	Pin compatible

LT8625S LQFN Package

SP model: Optional heat sink attachment for high ambient temperature applications.
Possible Exposed Die Top Package (Exposed Pad on Top)
No heat sink is required to achieve the rated output current.

We have evaluation boards available for each product, so please give them a try.

This time, we introduced Analog Devices' switching regulator, "Silent Switcher 3," which achieves unprecedented low noise.

If you are interested, please feel free to contact us.