MAX8760ETL+T Maxim Integrated Products, MAX8760ETL+T Datasheet - Page 35

• Manufacturer Part Number: MAX8760ETL+T
• Description: IC CNTRLR QUICK PWM 40-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8760ETL+T

• Applications: Controller, 6-bit VID AMD Mobile Turion™
• Voltage - Input: 4 ~ 28 V
• Number Of Outputs: 2
• Voltage - Output: 0.38 ~ 1.55 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 40-TQFN Exposed Pad
• Output Voltage: 0.375 V to 1.55 V
• Output Current: 4000 mA
• Mounting Style: SMD/SMT
• Switching Frequency: 550 KHz
• Maximum Operating Temperature: + 100 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Buck
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side, MOSFETs require
boost capacitors larger than 0.1µF. For these applica-
tions, select the boost capacitors to avoid discharging
the capacitor more than 200mV while charging the
high-side MOSFET’s gates:
where N is the number of high-side MOSFETs used for
one regulator, and Q
the MOSFET’s data sheet. For example, assume two
IRF7811W n-channel MOSFETs are used on the high
side. According to the manufacturer’s data sheet, a
single IRF7811W has a maximum gate charge of 24nC
(V
capacitance would be:
Selecting the closest standard value, this example
requires a 0.22µF ceramic capacitor.
The current-balance compensation capacitor (C
grates the difference between the main and secondary
current-sense voltages. The internal compensation resis-
tor (R
increasing the phase margin. This allows the dynamics of
the current-balance loop to be optimized. Excessively
large capacitor values increase the integration time con-
stant, resulting in larger current differences between the
phases during transients. Excessively small capacitor val-
ues allow the current loop to respond cycle-by-cycle but
can result in small DC current variations between the
phases. Likewise, excessively large resistor values can
also cause DC current variations between the phases.
Small resistor values reduce the phase margin, resulting
in marginal stability in the current-balance loop. For most
applications, a 470pF capacitor from CCI to the switching
regulator’s output works well.
Connecting the compensation network to the output
(V
voltage signal, especially during transients. To reduce
noise pickup in applications that have a widely distrib-
uted layout, it is sometimes helpful to connect the com-
pensation network to the quiet analog ground rather
than V
GS
OUT
= 5V). Using the above equation, the required boost
) allows the controller to feed-forward the output
CCI
OUT
Current-Balance Compensation (CCI)
.
= 20kΩ) improves transient response by
C
BST
Dual-Phase, Quick-PWM Controller for AMD
Mobile Turion 64 CPU Core Power Supplies
C
______________________________________________________________________________________
=
BST
GATE
2
200
=
x
24
N x Q
is the gate charge specified in
mV
200
nC
mV
GATE
=
0 24
.
µ
F
CCI
) inte-
Voltage positioning dynamically lowers the output volt-
age in response to the load current, reducing the
processor’s power dissipation. When the output is
loaded, an operational amplifier (Figure 5) increases
the signal fed back to the Quick-PWM controller’s feed-
back input. The adjustable amplification allows the use
of standard, current-sense resistor values, and signifi-
cantly reduces the power dissipated since smaller cur-
rent-sense resistors can be used. The load transient
response of this control loop is extremely fast, yet well
controlled, so the amount of voltage change can be
accurately confined within the limits stipulated in the
microprocessor power-supply guidelines.
The voltage-positioned circuit determines the load current
from the voltage across the current-sense resistors
(R
and output capacitors, as shown in Figure 10. The voltage
drop can be determined by the following equation:
where η
for voltage-positioning feedback, and η
number of active phases. When the slave controller is
disabled, the current-sense summation maintains the
proper voltage-positioned slope. Select the positive
input summing resistors so R
The nonadjustable minimum off-time one-shot and the
number of phases restrict the output voltage adjustable
range for continuous-conduction operation. For best
dropout performance, use the slower (200kHz) on-time
settings. When working with low input voltages, the
duty-factor limit must be calculated using worst-case
values for on- and off-times. Manufacturing tolerances
and internal propagation delays introduce an error to
the TON K factor. This error is greater at higher fre-
quencies (Table 6). Also, keep in mind that transient
response performance of buck regulators operated too
close to dropout is poor, and bulk output capacitance
must often be added (see the V
Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I
as much as it ramps up during the on-time (∆I
ratio h = ∆I
slew the inductor current higher in response to
SENSE
Minimum Input Voltage Requirements
SUM
= R
UP
CM
is the number of phases summed together
V
/∆I
VPS
= R
DOWN
A
VPS
CS
=
Setting Voltage Positioning
) connected between the inductors
A
and Dropout Performance
VPS LOAD SENSE
=
is an indicator of the ability to
η
I
η
TOTAL B
FBS
SUM F
= R
R
R
R
SAG
F
and R
TOTAL
equation in the
A
is the total
= R
UP
DOWN
B
). The
.
35
)