MAX15569 Datasheet, PDF (18/41 Pages) Maxim Integrated Products – 2-Phase/1-Phase QuickTune-PWM Controller with Serial I2C Interface

MAX15569
2-Phase/1-Phase QuickTune-PWM Controller with
Serial I²C Interface
Current Balance
The device integrates the difference between the current-
sense voltages and adjusts the on-time of the second
phase to maintain current balance. The current balance
relies on the accuracy of the current-sense signals across
the current-sense resistor, inductor DCR, or provided
by the power stage (MAX15515). With active current
balancing, the current mismatch is determined by the
current-sense element values and the offset voltage of the
transconductance amplifiers:
= I_OS(IBAL)
I_L= _MAIN - I_LSEC
V_OS(IBAL)
R _SENSE
where R_SENSE is the equivalent sense resistance across
CSP_, CSN_, and V_OS(IBAL) is the current-balance off-
set specification in the Electrical Characteristics section.
The worst-case current mismatch occurs immediately
after a load transient due to inductor value mismatches,
resulting in different dI/dt for the two phases. The time it
takes for the current-balance loop to correct the transient
imbalance depends on the mismatch between the
inductor values and switching frequency.
Current Limit
The current-limit circuit employs a “valley” current-sensing
algorithm that senses the voltage across the current-
sense inputs (CSP_ and CSN_). If the current-sense
signal (V_CSP2, V_CSN2 or V_CSP1, V_CSN1) of the selected
phase is above the current-limit threshold (V_ILIM), the
PWM controller does not initiate a new cycle for that
phase until its inductor current drops below the valley
current-limit threshold. Since only the valley current is
actively limited, the actual peak current is greater than
the current-limit threshold by an amount equal to 1/2 the
inductor ripple current:
I_LX(PEAK) =
I_LOAD
+
DI
2
I_{LX (VALLEY)}
=
I_LOAD
-
DI
2
where :
DI = ^{t ON(VIN - VOUT )}
L
where L is the inductance value, t_ON is the on-time of
the high-side MOSFET, V_OUT is the output voltage, and
V_IN is the input voltage. Therefore, the exact current-limit
characteristic and maximum load capability are functions
of the current-sense resistance, inductor value, and
battery voltage.
The positive valley current-limit threshold is preset for the
MAX15569. See the Electrical Characteristics section.
Current Limit Using Inductor DCR
or Sense Resistors
When using sense resistors or inductor DCR as
current-sensing elements, calculate the required sense
resistance (R_SENSE) with the following equation:
R
SENSE
=
V_LIM(MIN)
I_LX(VALLEY)
where I_LX(VALLEY) is the inductor valley current at OCP,
and V_ILIM(MIN) is 38mV ±3mV.
Carefully observe the PCB layout guidelines to ensure
that noise and trace errors do not corrupt the current-
sense signals seen by the current-sense inputs (CSP_,
CSN_).
Current Limit with the MAX15515 Current Sense
When using the current-sensing method of the MAX15515,
calculate the CSP_ - CSN_ resistor (R_{CSP_}) using the
following equation:
R _{CSP_}
=
V_LIM(MIN)
A×I_LX(VALLEY)
where A is 10^-5, I_LX(VALLEY) is the inductor valley current
at OCP, and V_ILIM(MIN) is 38mV ±3mV.
Current Monitoring (IMON)
The device includes a current monitoring function. A
simplified data-acquisition system is employed to convert
the analog signals from the current-sense inputs to 8-bit
values in the IMON register (see Figure 5). The ADC con-
verter filters the current-sense signal by averaging over
eight samples. The acquisition rate is 100µs. The content
of the IMON register is updated every 400µs.
The device includes a unidirectional transconductance
amplifier that sources current proportional to the posi-
tive current-sense voltage. The IMON output current is
defined by:
I_IMON = G_m(IMON) x ∑(V_{CSP_} - V_{CSN_})
= G_m(IMON) x I_LOAD x R_SENSE
where G_m(IMON) is the transconductance amplifier
gain, as defined in the Electrical Characteristics section
(5.12µA/mV typ).
An external resistor (R_IMON) between IMON and AGND
sets the current monitor output voltage:
V_IMON = I_IMON x R_IMON
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