## Overview of LTC3553-15 The **LTC3553-15** is a highly integrated Micropower Multifunction Power Management Integrated Circuit (PMIC) designed for portable, battery-powered applications. It combines a USB power manager, a Li-Ion/Polymer battery charger, a high-efficiency synchronous buck regulator, and a low dropout (LDO) linear regulator. The "-15" suffix specifically denotes a fixed LDO output voltage of **1.5V**. --- ### Key Internal Electronic Components The device integrates several critical functional blocks into a single 3mm × 3mm QFN package: | Component Block | Description | Key Specifications | | :--- | :--- | :--- | | **PowerPath™ Manager** | Manages power distribution between USB, Battery, and Load. | Priority given to external load; charges battery with leftover current. | | **Battery Charger** | Constant-Current/Constant-Voltage (CC/CV) charger. | Programmable charge current up to 200mA. | | **Buck Regulator** | High-efficiency synchronous step-down converter. | 200mA output current; 1.1MHz frequency. | | **LDO Regulator** | Low-noise linear regulator. | **Fixed 1.5V output**; 150mA current capability. | | **Always-On LDO** | Dedicated supply for keeping microcontrollers alive. | 3.3V fixed output; 10mA current. | --- ### Technical Specifications | Parameter | Value | | :--- | :--- | | **Input Voltage (VBUS)** | 4.35V to 5.5V | | **Battery Voltage Range** | Up to 4.2V | | **Buck Efficiency** | Up to 90% | | **Quiescent Current** | 12µA (Battery mode, all outputs on) | | **Package** | 20-lead 3mm × 3mm QFN | --- ### Pin Configuration & External Parts To function correctly, the LTC3553-15 requires several external passive components: 1. **Inductor (L):** Typically a **2.2µH to 10µH** inductor is required for the buck regulator to manage current ripple. 2. **Capacitors (C):** Ceramic capacitors (X5R or X7R) are used for $V_{BUS}$, $V_{OUT}$, and $V_{BAT}$ to ensure stability and reduce noise. 3. **Programming Resistor ($R_{PROG}$):** A resistor connected from the PROG pin to GND sets the maximum battery charging current. --- ### Application Circuit Example (Simplified) ```text [USB/5V] ----> [VBUS Pin] | [LTC3553-15 PMIC] ----> [VOUT1: Buck (Adjustable)] | ----> [VOUT2: LDO (Fixed 1.5V)] [Li-Ion] <---> [VBAT Pin] ``` ### Main Use Cases * Wearable health monitors * Low-power handheld medical devices * Small IoT sensors * Fitness trackers