LTC3526/LTC3526B function and working principle and component selection

Crystal oscillator

Mobile phone crystal 3.2*2.5mm 3225 26M (26.000MHZ) 7.5PF 10PPM 20PPM 30PPM

Single chip microcomputer STM32L151CCU6

The internal structure of the LTC3526/LTC3526B and its external component connections in the application are shown below.

Both ICs feature separate startup oscillators that can be activated at an input voltage as low as 0.85V. The chip provides a soft-start function and limits inrush current during startup and normal operation. Once the input or output voltage exceeds 1.4V, the IC enters normal operating mode. When the output voltage is higher than the input voltage by 0.24V, the IC switches to self-powering from the output, allowing for the use of a smaller input capacitor. Even when VIN drops to 0.5V, the IC continues to operate with the duty cycle clamped at 90%. During the soft-start phase, the inductor current ramps up from 0 to 700mA within 0.5ms. In off mode, the soft-start circuit resets. For 60ns, the peak switch current is limited to 700mA. A zero-current comparator monitors the inductor current and turns off the synchronous rectifier when the current drops to about 30mA, improving efficiency under light loads.

An anti-ringing circuit is connected across the external inductor, helping to suppress high-frequency ringing on the SW pin and reduce EMI radiation. The LTC3526/LTC3526B allow the actual output to be disconnected. During the IC's off period, VOUT can be set to 0V without drawing current from the input, and VOUT can rise freely without reverse current flowing into the battery.

The LTC3526 automatically enters burst mode operation at light loads, switching back to fixed-frequency PWM mode when the load increases. The LTC3526B operates at a frequency of 1MHz and disables pulse skipping under very light loads. Even when VIN is greater than VOUT, the LTC3526/LTC3526B maintain voltage regulation, although with lower efficiency and reduced maximum output current. If the SHDN pin voltage is below 0.3V, the IC turns off; if it’s above 0.8V, the IC is enabled. When the die junction temperature exceeds 160°C, the IC enters shutdown mode and re-enables once the temperature drops below 145°C.

Ll recommends using a 3.3–6.8μH chip inductor with low equivalent series resistance (ESR). The allowable inductor current ripple (peak-to-peak) is typically 30% of the maximum inductor current. The minimum inductor value is calculated using the following formula:

In the application circuit of the LTC3526/LTC3526B, the input capacitor CIN can be a low-ESR 1μF ceramic capacitor, while the output capacitor COUT should be a 4.7–10μF low-ESR multilayer ceramic capacitor.

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