PM6685 datasheet(33/52 Pages) STMICROELECTRONICS

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PM6685

Monitoring and protections

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8.5

Design guidelines

The design of a switching section starts from two parameters:

●

Input voltage range: in notebook applications it varies from the minimum battery

voltage, VINmin to the AC adapter voltage, VINmax.

●

Maximum load current: it is the maximum required output current, ILOAD(max).

8.6

Switching frequency

It’s possible to set 3 different working frequency ranges for the two sections:

200kHz/300kHz, 400kHz/500kHz, 600kHz/700kHz with FSEL pin.

Switching frequency mainly influences two parameters:

●

Inductor size: for a given saturation current and RMS current, greater frequency allows

to use lower inductor values, which means smaller size.

●

Efficiency: switching losses are proportional to frequency. High frequency generally

involves low efficiency.

8.7

Inductor selection

Once that switching frequency is defined, inductor selection depends on the desired

inductor ripple current and load transient performance.

Low inductance means great ripple current and could generate great output noise. On the

other hand, low inductor values involve fast load transient response.

A good compromise between the transient response time, the efficiency, the cost and the

size is to choose the inductor value in order to maintain the inductor ripple current

∆IL

between 20% and 50% of the maximum output current ILOAD(max). The maximum

∆IL

occurs at the maximum input voltage. With this considerations, the inductor value can be

calculated with the following relationship:

Table 10.

Protections and operatives modes

Mode

Conditions

Description

Overvoltage

protection

OUT5/OUT3 > 115% of the

nominal value

LGATE5/LGATE3 pin is forced high, LDO5 remains

active. Exit by a power on reset or toggling SHDN or

EN5/EN3

Undervoltage

protection

OUT5/OUT3 < 70% of the

nominal value

LGATE5/LGATE3 is forced high after the soft end

mode, LDO5 remains active. Exit by a power on reset

or toggling SHDN or EN5/EN3

Thermal

shutdown

TJ > +150°C

All circuitry off. Exit by a POR on VIN or toggling

SHDN.

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