MAX17009 Datasheet, PDF (36/45 Pages) Maxim Integrated Products – AMD Mobile Serial VID Dual-Phase Fixed-Frequency Controller

AMD Mobile Serial VID Dual-Phase
Fixed-Frequency Controller
L
=
⎛
⎝⎜
V_IN − V_OUT
f_SWI_LOAD(MAX)LIR
⎞
⎠⎟
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
where I_LOAD(MAX) is the maximum current per phase,
and f_SW is the switching frequency per phase.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. If using a
swinging inductor (where the inductance decreases lin-
early with increasing current), evaluate the LIR with
properly scaled inductance values. For the selected
inductance value, the actual peak-to-peak inductor rip-
ple current (ΔI_INDUCTOR) is defined by:
( ) _{ΔIINDUCTOR}
=
V_OUT V_IN − V_OUT
V_INf_SWL
Ferrite cores are often the best choice, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I_PEAK):
I_PEAK
=
⎛
⎝⎜
I_LOAD(MAX)
η_PH
⎞
⎠⎟
+
⎛
⎝⎜
ΔI_INDUCTOR
2
⎞
⎠⎟
Peak-Inductor Current Limit (ILIM)
The MAX17009 overcurrent protection employs a peak
current-sensing algorithm that uses either current-
sense resistors or the inductor’s DCR as the current-
sense element (see the Current Sense section). Since
the controller limits the peak inductor current, the maxi-
mum average load current is less than the peak cur-
rent-limit threshold by an amount equal to half the
inductor ripple current. Therefore, the maximum load
capability is a function of the current-sense resistance,
inductor value, switching frequency, and input-to-out-
put voltage difference. When combined with the output
undervoltage-protection circuit, the system is effectively
protected against excessive overload conditions.
The peak current-limit threshold is set by voltage differ-
ence between ILIM and REF using an external resistor-
divider:
V_CS(PK) = V_CSP_ - V_CSN_ = 0.05 x (V_REF - V_ILIM)
I_LIMIT(PK) = V_CS(PK) / R_SENSE
where R_SENSE is the resistance value of the current-
sense element (inductors’ DCR or current-sense resis-
tor), and I_LIMIT(PK) is the desired peak current limit (per
phase). The peak current-limit threshold voltage-adjust-
ment range is from 10mV to 50mV.
Output-Capacitor Selection
The output filter capacitor must have low-enough ESR
to meet output ripple and load-transient requirements.
In CPU V_CORE converters and other applications where
the output is subject to large-load transients, the output
capacitor’s size typically depends on how much ESR is
needed to prevent the output from dipping too low
under a load transient. Ignoring the sag due to finite
capacitance:
( ) ^RESR ^{+ R}PCB
≤ ^VSTEP
ΔI_LOAD(MAX)
In non-CPU applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR. When operating multiphase systems out-of-
phase, the peak inductor currents of each phase are
staggered, resulting in lower output-ripple voltage
(V_RIPPLE) by reducing the total inductor ripple current.
For nonoverlapping, multiphase operation (V_IN ≥ V_OUT),
the maximum ESR to meet the output-ripple-voltage
requirement is:
( ) ^RESR
⎡
≤⎢
⎣⎢
V_INf_SWL
V_IN − V_OUT V_OUT
⎤
⎥VRIPPLE
⎦⎥
where f_SW is the switching frequency per phase. The
actual capacitance value required relates to the physi-
cal size needed to achieve low ESR, as well as to the
chemistry of the capacitor technology. Thus, the
capacitor selection is usually limited by ESR and volt-
age rating rather than by capacitance value (this is true
of polymer types).
The capacitance value required is determined primarily
by the output transient-response requirements. Low
inductor values allow the inductor current to slew faster,
replenishing charge removed from or added to the out-
put filter capacitors by a sudden load step. Therefore,
the amount of output soar when the load is removed is
a function of the output voltage and inductor value. The
minimum output capacitance required to prevent over-
shoot (V_SOAR) due to stored inductor energy can be
calculated as:
( ) _{COUT ≥}
ΔI_LOAD(MAX) ²L
2V_OUTV_SOAR
36 ______________________________________________________________________________________