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PM6675AS Datenblatt(PDF) 35 Page - STMicroelectronics

Teilenummer PM6675AS
Bauteilbeschribung  High efficiency step-down controller with embedded 2 A LDO regulator
PDF  48 Pages
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Hersteller  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6675AS Datenblatt(HTML) 35 Page - STMicroelectronics

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PM6675AS
Application information
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Equation 28 a
Equation 28 b
Referring to the typical application schematic (figs. 1 and 23), the final expression is then:
Equation 29
Even if the switching frequency is theoretically independent from battery and output
voltages, parasitic parameters involved in power path (like MOSFETs on-resistance and
inductor DCR) introduce voltage drops responsible of a slight dependence on load current.
In addition, the internal delay is cause of a light dependence from input voltage.
8.1.1
Inductor selection
Once the switching frequency has been defined, the inductance value depends on the
desired inductor ripple current. Low inductance value means great ripple current that brings
to poor efficiency and great output noise. On the other hand a great current ripple is
desirable for fast transient response when a load step is applied.
Otherwise, great inductance brings to good efficiency but the transient response is critical,
especially if VINmin - Vout is little. Moreover a minimum output ripple voltage is necessary to
assure system stability and jitter-free operations (see Output capacitor selection paragraph).
The product of the output capacitor ESR multiplied by the inductor ripple current must be
taken into consideration. A good trade-off between the transient response time, the
efficiency, the cost and the size is to choose the inductance value in order to maintain the
inductor ripple current between 20 % and 50 % (usually 40 %) of the maximum output
current.
The maximum inductor ripple current,
∆I
LMAX, occurs at the maximum input voltage.
Table 10.
Typical values for switching frequency selection
R1 (k
Ω)R2 (kΩ)
Approx switching frequency (kHz)
330
11
250
330
13
300
330
15
350
330
18
400
330
20
450
330
22
500
IN
OSC
OSC
V
V
=
α
OUT
SNS
OUT
V
V
=
α
OSC
2
1
2
OSC
OSC
SW
K
1
R
R
R
K
f
⋅
+
=
α
=



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