MAX8770GTL+T Maxim Integrated Products, MAX8770GTL+T Datasheet - Page 40

• 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
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
In CPU V
the output is subject to large load transients, the output
capacitor’s size typically depends on how much ESR is
needed to prevent the output from dipping too low
under a load transient. Ignoring the sag due to finite
capacitance:
In non-CPU applications, the output capacitor’s size
often depends on how much ESR is needed to maintain
an acceptable level of output ripple voltage. The output
ripple voltage of a step-down controller equals the total
inductor ripple current multiplied by the output capaci-
tor’s ESR. When operating multiphase systems out-of-
phase, the peak inductor currents of each phase are
staggered, resulting in lower output ripple voltage by
reducing the total inductor ripple current. For multi-
phase operation, the maximum ESR to meet ripple
requirements is:
where η
f
capacitance value required relates to the physical size
needed to achieve low ESR, as well as to the chemistry
of the capacitor technology. Thus, the capacitor is usu-
ally selected by ESR and voltage rating rather than by
capacitance value (this is true of polymer types).
When using low-capacity ceramic filter capacitors,
capacitor size is usually determined by the capacity
needed to prevent V
problems during load transients. Generally, once
enough capacitance is added to meet the overshoot
requirement, undershoot at the rising load edge is no
longer a problem (see the V
in the Transient Response section).
CONFIDENTIAL INFORMATION – RESTRICTED TO INTEL
40
SW
is the switching frequency per phase. The actual
R
______________________________________________________________________________________
ESR
TOTAL
CORE
MAX8770/MAX8771/MAX8772 Dual-Phase, Quick-
(
R
≤
ESR
⎡
⎢
⎢
⎣
converters and other applications where
(
is the total number of active phases and
V
IN
+
−
Output Capacitor Selection
R
η
PCB
SAG
TOTAL OUT
V f
IN SW
)
and V
SAG
≤
V
L
∆
I
and V
LOAD MAX
)
SOAR
V
V
STEP
OUT
(
SOAR
⎤
⎥
⎥
⎦
from causing
V
)
RIPPLE
equations
For Quick-PWM controllers, stability is determined by
the value of the ESR zero relative to the switching
frequency. The boundary of instability is given by the
following equation:
where:
where C
total ESR, R
= R
R
put capacitors and sense resistors.
For a standard 300kHz application, the ESR zero fre-
quency must be well below 95kHz, preferably below
50kHz. Tantalum, Sanyo POSCAP, and Panasonic SP
capacitors in widespread use at the time of publication
have typical ESR zero frequencies below 50kHz. In the
standard application circuit, the ESR needed to support
a 30mV
330µF/2.5V Panasonic SP (type SX) capacitors in paral-
lel provide 1.5mΩ (max) ESR. With a 2mΩ droop and
0.5mΩ PC board resistance, the typical combined ESR
results in a zero at 30kHz.
Ceramic capacitors have a high ESR zero frequency,
but applications with significant voltage positioning can
take advantage of their size and low ESR. Do not put
high-value ceramic capacitors directly across the output
without verifying that the circuit contains enough voltage
positioning and series PC board resistance to ensure
stability. When only using ceramic output capacitors,
output overshoot (V
mum output capacitance requirement. Their relatively
low capacitance value can cause output overshoot
when stepping from full-load to no-load conditions,
unless a small inductor value is used (high switching
frequency) to minimize the energy transferred from
inductor to capacitor during load-step recovery. The
efficiency penalty for operating at 600kHz is approxi-
mately 5% when compared to the 300kHz circuit, pri-
marily due to the high-side MOSFET switching losses.
PCB
Output Capacitor Stability Considerations
CS
is the parasitic board resistance between the out-
), R
P-P
OUT
R
DROOP
EFF
SENSE
ripple is 30mV/(40A x 0.3) = 2.5mΩ. Four
is the total output capacitance, R
f
ESR
=
is the voltage-positioning gain, and
R
is the current-sense resistance (R
SOAR
=
ESR
f
®
ESR
2π
IMVP-6 LICENSEES
) typically determines the mini-
R
+
EFF OUT
≤
R
1
DROOP
f
C
SW
π
and
+
R
PCB
ESR
is the
CM