LTC2978 Datasheet, PDF (58/64 Pages) Linear Technology – Octal PMBus Power Supply Monitor and Controller with EEPROM

LTC2978
APPLICATIONS INFORMATION
Measuring Current
Odd numbered ADC channels may be used to measure
supply current. Set the ADC to high resolution mode to
configure for current measuring and improve sensitivity.
Note that no OV or UV faults or warnings are reported in
this mode, but telemetry is available from the READ_VOUT
command using the 11-bit signed mantissa plus 5-bit
signed exponent linear data format. Set the MFR_CONFIG
bit b[9] = 1 in order to enable high res mode. Note: Any
channel configured for ADC high res mode should be per-
manently commanded off, i.e., OPERATION register = ‘h00
(this is the factory default EEPROM value). The V_{OUT_ENn}
will assert low in this mode and cannot be used to control
a DC/DC. The V_DACPn output is also unavailable.
A circuit for measuring current is shown in Figure 18.
The balanced filter, R_CM, C_CM, rejects common mode and
differential mode noise from the inductor of the switching
DC/DC converter. The filter to the ADC is placed directly
across the inductor or a sense resistor in series with the
inductor. Note that the + input to the ADC must be limited
to <6V above ground. Select R_CM and C_CM such that the
corner frequency is ≤1/10 the DC/DC converter switching
frequency and ≤1/10 of the internal 62.5kHz clock of the
LTC2978’s ∆Σ ADC. This will assure that the DC value
at the (+) input to the ADC will be equal to the output
voltage with small ripple. Good values are R_CM = 100Ω,
C_CM ≥ 1µF. Keep R_CM ≤ 100Ω to minimize gain errors due
to ADC input resistance.
Many switching regulator control ICs derive AC inductor
current information from an RC network such as the one
formed by R1, C1, R2 and C2. The proper placement of
the balanced common mode filter for the ADC input is
also shown in Figure 18.
R_CM
R_CM
L
DCR
LTC2978
C_{CM +}
C_CM
– ^ADC
+
–
LOAD CURRENT
C1
R1
TO ⁺
CONTROLLER
IC _–
R2
C2
2978 F18
Figure 18. DCR Current Sensing Circuits
2978fa
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