MAX1540 Datasheet, PDF(35/49 Page) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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MAX1540 Datasheet, PDF (35/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

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Dual Step-Down Controllers with Saturation

Protection, Dynamic Output, and Linear Regulator

V+

*LDOIN

LDOON

INTERNAL LDOIN

OPTION BETWEEN

THE MAX1540/MAX1541

VL REG

AND REF

GATE DRIVER

AND ERROR

AMP

LDOOUT

INTERNAL VDD

OPTION BETWEEN

THE MAX1540/MAX1541

*VDD

FIXED 5V

*FBDLO

INTERNAL FBLDO

OPTION BETWEEN

THE MAX1540/MAX1541

0.2V

*MAX1541 ONLY.

Figure 13. Internal Linear-Regulator Functional Diagram

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 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.

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 current

(ILOAD) determines the thermal stresses and thus dri-

ves the selection of input capacitors, MOSFETs, and

other critical heat-contributing components.

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

making higher frequencies more practical.

â¢ Inductor operating point: This choice provides

trade-offs between size vs. efficiency and transient

response vs. output ripple. Low inductor values

provide better transient response and smaller phys-

ical size, but also result in lower efficiency and

higher output ripple due to increased ripple cur-

rents. The minimum practical inductor value is one

that causes the circuit to operate at the edge of crit-

ical 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.

When pulse skipping (SKIP low and light loads), the

inductor value also determines the load-current

value at which PFM/PWM switchover occurs.

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