PM6680A Datasheet, PDF (30/48 Pages) STMicroelectronics – Dual synchronous step-down controller with adjustable output voltages plus LDO

Design guidelines
9
Design guidelines
PM6680A
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, V_INmax.
● Maximum load current: it is the maximum required output current, I_LOAD(max).
9.1
Switching frequency
It's possible to set 3 different working frequency ranges for the two sections with FSEL pin
(Table 6).
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.
9.2
30/48
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 ∆I_L
between 20 % and 50 % of the maximum output current I_LOAD(max). The maximum ∆I_L
occurs at the maximum input voltage. With this considerations, the inductor value can be
calculated with the following relationship:
Equation 12
L = ^{VIN − VOUT} × ^VOUT
f_sw × ∆I_L
V_IN
where fsw is the switching frequency, V_IN is the input voltage, V_OUT is the output voltage and
∆I_L is the selected inductor ripple current.
In order to prevent overtemperature working conditions, inductor must be able to provide an
RMS current greater than the maximum RMS inductor current I_LRMS:
Equation 13
I_LRMS =
(I_LOAD (max))²
+
(∆I_L (max))²
12
Where ∆I_L(max) is the maximum ripple current: