LTC3738 Datasheet, PDF (18/32 Pages) Linear Technology – 3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning

LTC3738
APPLICATIO S I FOR ATIO
applied. Physically, if the capacitance value changes due
to applied voltage change, there is a concommitant piezo
effect which results in radiating sound! A load that draws
varying current at an audible rate may cause an attendant
varying input voltage on a ceramic capacitor, resulting in
an audible signal. A secondary issue relates to the energy
flowing back into a ceramic capacitor whose capacitance
value is being reduced by the increasing charge. The
voltage can increase at a considerably higher rate than the
constant current being supplied because the capacitance
value is decreasing as the voltage is increasing! Ceramic
capacitors, when properly selected and used however, can
provide the lowest overall loss due to their extremely low
ESR.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The steady-state output ripple (∆V_OUT) is determined by:
∆V_OUT
≈
∆I_RIPPLE^{⎛⎝⎜}ESR
+
1
8NfC_OUT
⎞
⎠⎟
where f = operating frequency of each stage, N is the
number of output stages, C_OUT = output capacitance and
∆I_L = ripple current in each inductor. The output ripple is
highest at maximum input voltage since ∆I_L increases
with input voltage. The output ripple will be less than 50mV
at max V_IN with ∆I_L = 0.4I_OUT(MAX) assuming:
C_OUT required ESR < N • R_SENSE
and
C_OUT > 1/(8Nf)(R_SENSE)
The emergence of very low ESR capacitors in small,
surface mount packages makes very small physical imple-
mentations possible. The ability to externally compensate
the switching regulator loop using the I_TH pin allows a
much wider selection of output capacitor types. The
impedance characteristics of each capacitor type is sig-
nificantly different than an ideal capacitor and therefore
requires accurate modeling or bench evaluation during
design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
18
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo and the Panasonic SP
surface mount types have a good (ESR)(size) product.
Once the ESR requirement for C_OUT has been met, the
RMS current rating generally far exceeds the I_RIPPLE(P-P)
requirement. Ceramic capacitors from AVX, Taiyo Yuden,
Murata and Tokin offer high capacitance value and very
low ESR, especially applicable for low output voltage
applications.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer sur-
face mount capacitors offer very low ESR also but have
much lower capacitive density per unit volume. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
choices are the AVX TPS, AVX TPSV, the KEMET T510
series of surface-mount tantalums or the Panasonic SP
series of surface mount special polymer capacitors avail-
able in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POS-CAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturer for other specific recommendations.
R_SENSE Selection for Output Current
Once the frequency and inductor have been chosen,
R_SENSE1, R_SENSE2, R_SENSE3 are determined based on the
required peak inductor current. The current comparator
has a maximum threshold of 75mV/R_SENSE and an input
common mode range of SGND to (1.1) • V_CC. The current
comparator threshold sets the peak inductor current,
yielding a maximum average output current I_MAX equal to
the peak value less half the peak-to-peak ripple current,
∆I_L.
Allowing a margin for variations in the IC and external
component values yields:
R_SENSE
=
N
50mV
I_MAX
The IC works well with values of R_SENSE from 0.001Ω to
0.02Ω.
3738f