MAX8770GTL+T Maxim Integrated Products, MAX8770GTL+T Datasheet - Page 39

• 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
• Switching frequency: This choice determines the
• Inductor operating point: This choice provides
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
where η
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, although powdered
iron is inexpensive and can work well at 200kHz. The
core must be large enough not to saturate at the peak
inductor current (I
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage due to MOSFET switching losses that are
proportional to frequency and V
quency is also a moving target, due to rapid
improvements in MOSFET technology that are mak-
ing higher frequencies more practical.
trade-offs between size vs. efficiency and transient
response vs. output noise. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output noise due to increased ripple current. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero
with every cycle at maximum load). Inductor values
lower than this grant no further size-reduction benefit.
The optimum operating point is usually found
between 20% and 50% ripple current.
L
TOTAL
MAX8770/MAX8771/MAX8772 Dual-Phase, Quick-
=
I
PEAK
η
TOTAL
is the total number of phases.
=
PEAK
______________________________________________________________________________________
⎛
⎜
⎝
⎛
⎜
⎝
f
SW LOAD MAX
I
LOAD MAX
):
η
I
V
TOTAL
IN
(
−
V
(
OUT
)
Inductor Selection
⎞
⎟
⎠
IN
)
LIR
⎛
⎜
⎝
1
2
. The optimum fre-
+
⎞
⎟
⎠
⎛
⎜
⎝
V
LIR
2
OUT
V
IN
⎞
⎟
⎠
⎞
⎟
⎠
The inductor ripple current impacts transient-response
performance, especially at low V
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output fil-
ter capacitors by a sudden load step. The amount of
output sag is also a function of the maximum duty fac-
tor, which can be calculated from the on-time and mini-
mum off-time. For a dual-phase controller, the
worst-case output sag voltage may be determined by:
where t
Electrical Characteristics table).
The amount of overshoot due to stored inductor energy
can be calculated as:
where η
The minimum current-limit threshold must be high
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
ley of the inductor current occurs at I
half the ripple current; therefore:
where η
and I
threshold voltage divided by the current-sense resistor
(R
current-limit threshold is 19.5mV.
V
SAG
SENSE
I
LIMIT LOW
LIMIT(LOW)
=
+
TOTAL
OFF(MIN)
). For the 22.5mV default setting, the minimum
2
TOTAL
∆
(
C
I
L I
LOAD(MAX)
OUT OUT
(
V
2
∆
SOAR
C
LOAD(MAX)
is the total number of active phases.
OUT
)
V
is the total number of active phases,
>
equals the minimum current-limit
is the minimum off-time (see the
⎛
⎜
⎝
≈
I
®
⎡
⎢
⎢
⎣
LOAD MAX
⎛
⎜
⎝
η
⎡
⎢ ⎢
⎢
⎣
2η
(
Setting the Current Limit
⎛
⎜
⎝
IMVP-6 LICENSEES
V
TOTAL
)
(
IN
2
V
∆
TOTAL OUT OUT
OUT SW
(
I
−
LOAD MAX
⎡
⎢
⎢
⎣
V
⎛
⎜
⎝
2
IN
Transient Response
V
V
T
V
OUT SW
OUT
IN
)
C
⎞
⎟
⎠
(
V
IN
IN
⎞
⎟ +
⎠
T
⎛
⎜
⎝
)
T
1
- V
SW
)
−
V
LOAD(MAX)
)
OUT
t
⎞
⎟ +
⎠
2
OFF MIN
⎞
⎟ −
⎠
L
LIR
2
(
differentials.
t
OFF MIN
2
⎞
⎟
⎠
t
OFF MIN
)
⎤
⎥
⎥
⎦
(
(
minus
)
⎤
⎥
⎥
⎦
)
39
⎤
⎥
⎥
⎦