Introduction

If you design long-life battery powered device you should pay attention at the power consumption of each IC not only in work mode, but also in standby mode, if device should work even in sleep mode.

If you use standard Li-Ion battery (3.2V-4.2V) and have 3.3V or 1.8V level MCU most likely you use buck-boost converter or buck converter to achieve stable powering voltage in all battery voltage range.

So, let's take a look at the issue using Texas Instruments high efficacy DC-DC voltage converter – TPS63020 (2.4MHz; 4A SW).

🔺 Issue Description

Let's assume your device enter in sleep/deep-sleep mode and main MCU and other stuff current is very small (typically in range 1u-100uA) and now quiescent current of the buck-boost DC-DC converter is most significant in overall value:

VIN = 5.066V	VOUT = 3.290V	Isupply = 8.02mA

Damn, Isupply=8.02mA which is too big and not appropriatefor battery-powered device, because if you use 1000mAh battery the device standby time will be only:

BatCapacity/Istandby = 1000mAh/12mA = 124.69h= 5.19 days

Then you will over discharge the battery, because it's always connected in most of modern devices.

🔧 Solution

Let's look at the datasheet and find section about Save Mode, as you can see to enter in this mode we should pull-down PS/SYNC pin. 

Let's compare what is the supply current when power save mode disabled (1 — HIGH) and enabled (0 — LOW).

PS/SYNC= VIN = 5.066V	PS/SYNC = 0V
Isupply = 8.02mA	Isupply = 27.8 uA (up to 50 uA)

As you can see difference is huge, like:

8.02mA/27.8uA = 288 times

That means if you use the same 1000mAh battery the device standby time will be:

BatCapacity/Istandby = 1000mAh/27.8uA = 35971h= 1498 days = 4 years

Output Voltage Ripple in Save Mode

Be careful, when TPS63020 enters in save mode output voltage increases by 2.5~3.5% with some ripple, it's necessary for heavy load. As you can see in normal mode VOUT=3.29V, but in save mode VOUT=3.425V, which is higher by 0.135V or by 4.1%.

VOUT Ripple = 0.176V	VOUT = 3.425V

Additional Ripple Measurement

Save Mode	Normal Mode

Statuses

Conclusions

• Always pay attention on quiescent current of the DC-DC converters if you're design long battery-life system
• Many similar Texas Instruments DC-DC converters have PS/SYNC (PS — Power Save) pin, so their behavior will be similar. When device in normal operation mode disable Save Mode, so you can normally supply heavy load, but when device goes sleep just enable Save Mode to decrease quiescent current
• For some DC-DC converters (like TPS63020) output voltage and ripple in save mode will be higher than in normal mode
• Some modern high-voltage DC-DC like LM5164 (100V; 1A;  up to 1MHz) converters have automatic save mode when load current is small