MAX1718 Maxim, MAX1718 Datasheet - Page 24

• Manufacturer Part Number: MAX1718
• Description: Notebook CPU Step-Down Controller for Intel Mobile Voltage Positioning IMVP-II
• Manufacturer: Maxim

Notebook CPU Step-Down Controller for Intel
Mobile Voltage Positioning (IMVP - II)
Table 6. Operating Mode Truth Table
The over/undervoltage protection features can compli-
cate the process of debugging prototype breadboards
since there are (at most) a few milliseconds in which to
determine what went wrong. Therefore, a test mode is
provided to disable the OVP, UVP, and thermal shut-
down features, and clear the fault latch if it has been
set. The PWM operates as if SKP/SDN were high (SKIP
mode). The NO FAULT test mode is entered by forcing
12V to 15V on SKP/SDN.
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
1) Input Voltage Range. The maximum value (V
2) Maximum Load Current. There are two values to con-
24
must accommodate the worst-case high AC adapter
voltage. The minimum value (V
for the lowest battery voltage after drops due to con-
nectors, fuses, and battery selector switches. If there
is a choice at all, lower input voltages result in better
efficiency.
sider. The peak load current (I
mines the instantaneous component stresses and
filtering requirements, and thus drives output capaci-
tor selection, inductor saturation rating, and the
design of the current-limit circuit. The continuous load
current (I
thus drives the selection of input capacitors,
V
12V to 15V
CC
SKP/SDN
______________________________________________________________________________________
Open
GND
V
or Open
CC
LOAD
) determines the thermal stresses and
Switching
Switching
Switching
High
High
DL
Design Procedure
NO FAULT Test Mode
IN(MIN)
LOAD(MAX)
Run (PWM, low noise)
Run (PFM/PWM)
) must account
Shutdown
No Fault
MODE
Fault
IN(MAX)
) deter-
)
3) Switching Frequency. This choice determines the
4) Inductor Operating Point. This choice provides trade-
5) The inductor ripple current also impacts transient-
Low-power shutdown state. DL is forced to V
OVP. I
Test mode with faults disabled and fault latches cleared, includ-
ing thermal shutdown. Otherwise, normal operation, with auto-
matic PWM/PFM switchover for pulse-skipping at light loads.
Low-noise operation with no automatic switchover. Fixed-fre-
quency PWM action is forced regardless of load. Inductor cur-
rent reverses at light load levels.
Operation with automatic PWM/PFM switchover for pulse-skip-
ping at light loads.
Fault latch has been set by OVP, UVP, or thermal shutdown.
Device will remain in FAULT mode until V
SKP/SDN is forced low.
MOSFETs, and other critical heat-contributing com-
ponents. Modern notebook CPUs generally exhibit
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 pro-
portional to frequency and V
cy is also a moving target, due to rapid improvements
in MOSFET technology that are making higher frequen-
cies more practical.
offs between size and efficiency. Low inductor val-
ues cause large ripple currents, resulting in the
smallest size, but poor efficiency and high output
noise. The minimum practical inductor value is one
that causes the circuit to operate at the edge of criti-
cal conduction (where the inductor current just touch-
es zero with every cycle at maximum load). Inductor
values lower than this grant no further size-reduction
benefit.
The MAX1718’s pulse-skipping algorithm initiates
skip mode at the critical conduction point. So, the
inductor operating point also determines the load-
current value at which PFM/PWM switchover occurs.
The optimum point is usually found between 20%
and 50% ripple current.
response performance, especially at low V
differentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load
LOAD
CC
+ I
= I
DD
LOAD(MAX)
= 2µA typ.
COMMENT
✕
80%.
IN 2
. The optimum frequen-
CC
power is cycled or
DD
, enforcing
IN
- V
OUT