MAX8770GTL+T Maxim Integrated Products, MAX8770GTL+T Datasheet - Page 24

• Manufacturer Part Number: MAX8770GTL+T
• Description: IC CTLR PS 2/1PH QUICK PWM 40QFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8770GTL+T

• Applications: Controller, Intel IMVP-6
• Voltage - Input: 4 ~ 26 V
• Number Of Outputs: 1
• Voltage - Output: 0.125 ~ 1.5 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PWM Controller for IMVP-6+ CPU Core Power Supplies
CONFIDENTIAL INFORMATION – RESTRICTED TO INTEL
Without active current-balance circuitry, the current
matching between phases depends on the MOSFETs’
on-resistance (R
matching, and inductance matching. For example, vari-
ation in the low-side MOSFET on-resistance (ignoring
thermal effects) results in a current mismatch that is
proportional to the on-resistance difference:
However, mismatches between on-times, off-times, and
inductor values increase the worst-case current imbal-
ance, making it impossible to passively guarantee
accurate current balancing.
The MAX8770/MAX8771/MAX8772 integrate the differ-
ence between the current-sense voltages and adjust the
on-time of the secondary phase to maintain current bal-
ance. The current balance now relies on the accuracy of
the current-sense resistors instead of the inaccurate, ther-
mally sensitive on-resistance of the low-side MOSFETs.
With active current balancing, the current mismatch is
determined by the current-sense resistor values and the
offset voltage of the transconductance amplifiers:
where R
rent-balance offset specification in the Electrical
Characteristics table .
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 the current-balance loop to correct the transient
imbalance depends on the mismatch between the
inductor values and switching frequency.
The current-limit circuit employs a unique “valley” cur-
rent-sensing algorithm that uses current-sense resistors
between the current-sense inputs (CSP_ to CSN12 for
MAX8771, CSP_ to CSN_ for MAX8770/MAX8772) as
the current-sensing elements. If the current-sense sig-
nal of the selected phase is above the current-limit
threshold, the PWM controller does not initiate a new
cycle until the inductor current of the selected phase
drops below the valley current-limit threshold. When
either phase trips the current limit, both phases are
effectively current limited since the interleaved con-
troller does not initiate a cycle with either phase.
24
______________________________________________________________________________________
SENSE
I
OS IBAL
MAX8770/MAX8771/MAX8772 Dual-Phase, Quick-
I
(
MAIN
= R
DS(ON)
)
−
=
I
CM
SEC
I
LMAIN
= R
), thermal ballasting, on/off-time
=
I
MAIN
−
CS
I
LSEC
and V
⎡
⎢
⎢
⎣
1
−
=
⎛
⎜
⎝
R
V
R
OS(IBAL)
R
OS IBAL
MAIN
SEC
SENSE
Current Balance
(
Current Limit
⎞
⎟
⎠
⎤
⎥
⎥
⎦
)
is the cur-
Since only the valley current is actively limited, the actu-
al peak current is greater than the current-limit threshold
by an amount equal to the inductor ripple current.
Therefore, the exact current-limit characteristic and
maximum load capability are a function of the current-
sense resistance, inductor value, and battery voltage.
When combined with the UVP circuit, this current-limit
method is effective in almost every circumstance.
The positive current-limit threshold is fixed internally at
22.5mV. There is also a negative current limit that pre-
vents excessive reverse inductor currents when V
is sinking current. The negative current-limit threshold
is set at -30mV. When a phase drops below the nega-
tive current limit, the controller immediately activates an
on-time pulse—DL turns off, and DH turns on—allowing
the inductor current to remain above the negative cur-
rent threshold.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the cur-
rent-sense signals seen by the current-sense inputs
(CSP_, CSN_). For the MAX8771, where the negative
current-sense returns are to a common pin, it is recom-
mended that the current-sense elements (sense resis-
tor or inductor DCR) be placed close to each other to
minimize any voltage differences that might arise due
to trace impedance between the two common nodes.
The MAX8770/MAX8771/MAX8772 include a transcon-
ductance amplifier for adding gain to the voltage-posi-
tioning sense path. The amplifier’s input is generated
by summing the current-sense inputs, which differen-
tially sense the voltage across either current-sense
resistors or the inductor’s DCR. The amplifier’s output
connects directly to the regulator’s voltage-positioned
feedback input (FB), so the resistance between FB and
the output-voltage sense point determines the voltage-
positioning gain:
where the target voltage (V
Nominal Output Voltage Selection section, and the FB
amplifier’s output current (I
sum of the current-sense voltages:
where V
sense voltage, and G
in the Electrical Characteristics table.
CS
= V
Feedback Adjustment Amplifiers
V
CSP
OUT
I
FB
=
- V
= V
®
m(FB)
G
m FB
CSN
IMVP-6 LICENSEES
Voltage-Positioning Amplifier
TARGET
(
is typically 600µS as defined
)
TARGET
is the differential current-
FB
η
X
∑
PH
=
) is determined by the
1
V
- R
(Steady-State Droop)
CSX
FB
) is defined in the
I
FB
OUT