MAX1717 Maxim, MAX1717 Datasheet - Page 24

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MAX1717

Manufacturer Part Number
MAX1717
Description
Dynamically Adjustable / Synchronous Step-Down Controller for Notebook CPUs
Manufacturer
Maxim
Datasheet

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Table 5. Operating Mode Truth Table
Dynamically Adjustable, Synchronous
Step-Down Controller for Notebook CPUs
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-
3) Switching Frequency. This choice determines the
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,
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
V
LOAD
12V to 15V
SKP/SDN
CC
______________________________________________________________________________________
GND
Float
V
or Float
CC
= I
LOAD
LOAD(MAX)
) determines the thermal stresses and
Switching
Switching
Switching
· 80%.
High
High
DL
Design Procedure
IN(MIN)
LOAD(MAX)
Run (PWM, low noise)
normal operation)
Run (PFM/PWM,
) must account
Shutdown
No Fault
MODE
Fault
IN(MAX)
) deter-
)
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.
Normal operation with automatic PWM/PFM switchover for
pulse-skipping 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.
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 vs. efficiency. Low inductor values
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 critical conduction
(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 MAX1717’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
step. The amount of output sag is also a function of
the maximum duty factor, which can be calculated
from the on-time and minimum off-time:
CC
+ I
DD
= 2µA typ.
COMMENT
IN 2
. The optimum frequen-
CC
power is cycled or
DD
, enforcing
IN
- V
OUT

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