MAX1544 Datasheet, PDF(31/42 Page) Maxim Integrated Products – Dual-Phase, Quick-PWM Controller for AMD Hammer CPU Core Power Supplies

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MAX1544 Datasheet, PDF (31/42 Pages) Maxim Integrated Products – Dual-Phase, Quick-PWM Controller for AMD Hammer CPU Core Power Supplies

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Dual-Phase, Quick-PWM Controller for

AMD Hammer CPU Core Power Supplies

Power-On Reset

Power-on reset (POR) occurs when VCC rises above

approximately 2V, resetting the fault latch, activating

boot mode, and preparing the PWM for operation. VCC

undervoltage lockout (UVLO) circuitry inhibits switch-

ing, and forces the DL gate driver high (to enforce out-

put overvoltage protection). When VCC rises above

4.25V, the DAC inputs are sampled and the output volt-

age begins to slew to the target voltage.

For automatic startup, the battery voltage should be

present before VCC. If the Quick-PWM controller

attempts to bring the output into regulation without the

battery voltage present, the fault latch trips. Toggle the

SHDN pin to reset the fault latch.

Input Undervoltage Lockout

During startup, the VCC UVLO circuitry forces the DL

gate driver high and the DH gate driver low, inhibiting

switching until an adequate supply voltage is reached.

Once VCC rises above 4.25V, valid transitions detected

at the trigger input initiate a corresponding on-time

pulse (see the On-Time One-Shot (TON) section). If the

VCC voltage drops below 4.25V, it is assumed that

there is not enough supply voltage to make valid deci-

sions. To protect the output from overvoltage faults, the

controller activates the shutdown sequence.

Multiphase Quick-PWM

Design Procedure

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:

â¢ Input voltage range: The maximum value

(VIN(MAX)) must accommodate the worst-case high

AC adapter voltage. The minimum value (VIN(MIN))

must account for the lowest input voltage after drops

due to connectors, fuses, and battery selector

switches. If there is a choice at all, lower input volt-

ages result in better efficiency.

â¢ Maximum load current: There are two values to

consider. The peak load current (ILOAD(MAX)) deter-

mines the instantaneous component stresses and fil-

tering requirements, and thus drives output capacitor

selection, inductor saturation rating, and the design

of the current-limit circuit. The continuous load cur-

rent (ILOAD) determines the thermal stresses and

thus drives the selection of input capacitors,

MOSFETs, and other critical heat-contributing com-

ponents. Modern notebook CPUs generally exhibit

ILOAD = ILOAD(MAX) Ã 80%.

For multiphase systems, each phase supports a

fraction of the load, depending on the current bal-

ancing. When properly balanced, the load current is

evenly distributed among each phase:

ILOAD(PHASE)

ILOAD

Î· TOTAL

where Î·TOTAL is the total number of active phases.

â¢ Switching frequency: This choice determines the

basic trade-off between size and efficiency. The

optimal frequency is largely a function of maximum

input voltage due to MOSFET switching losses that

are proportional to frequency and VIN2. The opti-

mum frequency is also a moving target, due to rapid

improvements in MOSFET technology that are mak-

ing higher frequencies more practical.

â¢ Inductor operating point: This choice provides

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 out-

put noise due to increased ripple current. The mini-

mum 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

optimum operating point is usually found between

20% and 50% ripple current.

Inductor Selection

The switching frequency and operating point (% ripple

current or LIR) determine the inductor value as follows:

Î·

TOTAL

ï£«

ï£ï£¬

VIN â VOUT

fSWILOAD(MAX)LIR

ï£¶

ï£¸ï£·

ï£«

ï£ï£¬

VOUT

VIN

ï£¶

ï£¸ï£·

where Î·TOTAL is the total number of phases.

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 (IPEAK):

IPEAK

ï£«

ï£¬

ï£

ILOAD(MAX)

Î· TOTAL

ï£¶

ï£·

ï£¸

ï£«ï£ï£¬1

LIR ï£¶

2 ï£¸ï£·

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