ISL9120 Buck-Boost Switching Regulator
Handheld Power Products
Welcome to the ISL9120 buck-boost switching regulator product overview. Today, we will be talking about how a buck-boost regulator adds value in a mobile/wearable device and introduce Intersil’s ISL9120 buck-boost switching regulator which is highly compact, with excellent efficiency. We will look at some of its key features, and point out some ideal applications for this part in the wearables and mobile space.
Why a Buck-Boost Is Needed In a Mobile Device?
How does a buck-boost add value in a mobile system? An Li-ion battery ranges from 4.2V to 2.7V and in a system to be powered by 3.3V, the buck-boost helps in utilizing a broader range of the battery. Multiple blocks in a mobile system are powered by an LDO such as Wi-Fi, Bluetooth, SD card, and LCD module. A buck-boost helps in optimizing the efficiency in these application as a pre-regulator and provides better system efficiency. When the battery voltage is close to 3.3V, a burst current can cause a local node voltage to droop to 2.7V, which is below the regulation point and hence can cause the application to shut down. This can be avoided by using a buck-boost as a pre-regulator.
As discussed in the previous slide, when multiple applications are being used together, there can be narrow pulse high current as high as 4A. As an Li-ion battery inherently has ~100 to 200mΩ ESR, the voltage drop across it can cause the local node voltage to droop below 2.85V and will cause the EMMC to shut off. This behavior is highly undesirable. If we use a buck-boost with VOUT of 2.9V to power the LDO, the buck-boost will block this perturbation without affecting the input to the LDO, hence avoiding the brownout issue.
Compact High Efficiency Buck-Boost with Bypass
The ISL9120 is a highly efficient and compact buck-boost regulator. ISL9120 operates from 1.8V to 5.5V. The output voltage is adjustable from 1V to 5.2V which provides flexibility to cover a variety of design needs. The adaptive PFM operation helps in getting excellent efficiency and low output ripple. This part is capable of delivering 800mA current with 2.5V in and 3.3V out. The ISL9120 is offered in a 1.4mm x 1.4mm WLCSP package. During system stay alive conditions when regulation is not a must, the part can be put in forced bypass mode, in which the power consumption reduces to ultra-low quiescent current of <0.5μA.
Fixed vs. Adaptive Current Limit Schemes
The adaptive PFM current limit scheme helps in reducing both VOUT ripple and increasing the efficiency as compared to the fixed current scheme. As the RMS current in the inductor is lower for a low load condition, it helps lower the conduction losses.
To optimize efficiency across the output current range, the ISL9120 implements a multi-level current limit scheme with 32 levels between 350mA and 2A. The transition from one level to the other is determined by the number of pulses in a PFM burst (pulse count). The lower peak current also helps in attaining smaller VOUT ripple.
At a given peak current limit level, the pulse count increases as the output current increases. When the pulse count reaches the upper threshold at the existing current limit, the current limit will switch to the next higher level. Similarly, if the pulse count reaches the lower threshold at the existing current limit, the device will switch to the next lower level of peak current limit. If the pulse count reaches the upper threshold at the highest current limit, the current limit will not rise any further.
ISL9120 Efficiency Plot
The ISL9120 offers excellent efficiency for both low load and high load conditions. The adaptive PFM operation helps in attaining up to 98% efficiency at higher load and >86% at lower load conditions. This ensures less power drain and less heat buildup which extends the battery life and saves board space by eliminating the need for external heat sinks.
The ISL9120 with its excellent efficiency and compact size is highly suitable for wearables/mobile and IoT devices. Applications like the heart rate sensor and the display in wearables require 3.3V to 3.6V input and a buck-boost is a good fit for it. Smartphones and tablets have the Wi-Fi module and LCD module which have similar VIN requirements, making the ISL9120 favorable for these applications.
With high efficiency, low power consumption and an ultra-small PCB footprint, the ISL9120’s adaptive PFM current limit architecture helps to extend battery life and simplify design. The wide input and output voltage ranges provide flexibility to cover a variety of design needs.