PAM8906

Document number: DS44739 Rev. 1 - 2

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December 2022

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PAM8906

Application Information (continued)

Boost Regulator

The PAM8906 boost regulator employs conventional current-mode control methodology with the following features:



Under-Voltage Lockout (UVLO): When the Boost power supply VIN drops below UVLO value (1.5V Typ), the Boost regulator will be switched

off.



Over Temperature Protection (OTP): When PAM8906 die temperature is higher than OTP threshold (+150°C Typ

), the Boost regulator will

be switched off. Hysteresis is set at +30°C.



VOUT Under-Voltage (UV) Protection: When the Boost regulator output voltage is lower than the pre-set threshold, it will trigger VOUT UV

protection, and the device will go to hiccup mode in order to reduce power supply current.



VOUT Over-Voltage (OV) Protection: When the Boost regulator output voltage is higher than the pre-set threshold, it will trigger VOUT OV

protection, and the device will switch off until VOUT OV condition is removed.



Soft-Start: The Boost regulator has a typical soft-start time of 770

μs. Soft-start helps to reduce in-rush currents and voltage over-shoot at

start-up.



Over Current Protection (OCP): The Boost regulator has a built-in cycle-by-cycle over-current protection.

PAM8906 has designed the boost regulator as current-mode controller with two control loops, which work together in maintaining a constant

output voltage and supply the required load current. The inner current control loop provides cycle-by-cycle current limiting, while the output control

loop provides output voltage control. When the boost converter turns on with the DIN input, the NMOS switch will turn on and the inductor current

ramps up to its peak value, approximately 1A nominally.

The current comparator turns off the NMOS switch for a fixed period of time to allow energy to be transferred to the output capacitor. When the

voltage on the output capacitor equals or exceeds the desired output voltage, the current loop will disable until the load discharges the output

capacitor to a voltage lower than the desired output voltage. Every time the output voltage falls below the desired value, the switching cycle starts

and continues until the desired value is reached. The constant switching resulting in the charging and discharging of the output capacitor causes a

ripple on the output voltage. The ripple on the output voltage depends on the external component parameters, such as the value of external

capacitor, its ESR etc.

In PAM8906, when logic high asserted on the DIN pin, the boost regulator will be enabled. However, the Piezo driver output cannot be enabled

until the output voltage reaches its nominal set point (total soft start time around 770

μs). This ensures the output voltage rises quickly to the

necessary drive voltage for the Piezo.

The boost regulator is optimized to work with the external components as shown in the Typical Application Circuits. It is crucial to select an

appropriate inductor value for the Boost converter to operate efficiently with the expected transient behavior and loop stability. Typical, an inductor

with saturation current rating of 2A is recommended.

The inductor selection for PAM8906 depends on the VIN value and the VOUT version chosen for a given application. Although minimum VIN is 2.1V,

if the input supply droops down to 1.8V, this can still be supported with a lower inductor value. Refer to the table below:

—

Recommend Inductor

VIN Min

L@VOUT = 10V

L@VOUT = 12V

L@VOUT = 18V

1.8V

0.47µH

0.22µH

0.22µH

2.1V

1.0µH

0.47µH

0.47µH