MAX8733EEI datasheet(20/32 Pages) MAXIM

No de pièce	MAX8733EEI

Description	High-Efficiency, Quad-Output, Main Power-Supply Controllers for Notebook Computers

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Fabricant	MAXIM [Maxim Integrated Products]

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20 / 32 page

High-Efficiency, Quad-Output, Main Power-

Supply Controllers for Notebook Computers

20

______________________________________________________________________________________

Reference and Linear Regulators

(REF, LDO5, and LDO3)

The 2V reference (REF) is accurate to ±1% over tem-

perature, making REF useful as a precision system

reference. Bypass REF to GND with a 0.22µF (min)

capacitor. REF can supply up to 100µA for external

loads. However, if extremely accurate specifications for

both the main output voltages and REF are essential,

avoid loading REF. Loading REF reduces the LDO5,

LDO3, OUT5, and OUT3 output voltages slightly

because of the reference load-regulation error.

Two internal regulators produce 5V (LDO5) and 3.3V

(LDO3). LDO5 provides gate drive for the external

MOSFETs and powers the PWM controller, logic, refer-

ence, and other blocks within the device. The LDO5

regulator supplies a total of 100mA for internal and

external loads, including MOSFET gate drive, which

typically varies from 10mA to 50mA, depending on

switching frequency and the external MOSFETs. LDO3

powers up when the reference (REF) is in regulation,

and supplies up to 100mA for external loads. Bypass

LDO5 and LDO3 with a minimum 4.7µF load; use an

additional 1µF per 5mA of internal and external load.

When the 5V main output voltage is above the LDO5

bootstrap-switchover threshold, an internal 1.4Ω p-chan-

nel MOSFET switch connects OUT5 to LDO5 while simul-

taneously shutting down the LDO5 linear regulator.

Similarly, when the 3.3V main output voltage is above the

LDO3 bootstrap-switchover threshold, an internal 1.5Ω

p-channel MOSFET switch connects OUT3 to LDO3 while

simultaneously shutting down the LDO3 linear regulator.

These actions bootstrap the device, powering the internal

circuitry and external loads from the output SMPS volt-

ages, rather than through linear regulators from the bat-

tery. Bootstrapping reduces power dissipation due to

gate charge and quiescent losses by providing power

from a 90%-efficient switch-mode source, rather than

from a much-less-efficient linear regulator.

Current-Limit Circuit (ILIM_)

The current-limit circuit employs a “valley” current-sens-

ing algorithm. The MAX8734 uses the on-resistance of

the synchronous rectifier, while the MAX8732/MAX8733

use a discrete resistor in series with the source of the

synchronous rectifier as a current-sensing element. If the

magnitude of the current-sense signal at CS_

(MAX8732/MAX8733)/LX_ (MAX8734) is above the cur-

rent-limit threshold, the PWM is not allowed to initiate a

new cycle (Figure 7). The actual peak current is greater

than the current-limit threshold by an amount equal to the

inductor ripple current. Therefore, the exact current-limit

characteristic and maximum load capability are a func-

tion of the current-limit threshold, inductor value, and

input and output voltage.

For the MAX8732/MAX8733, connect CS_ to the junction

of the synchronous rectifier source and a current-sense

resistor to GND. With a current-limit threshold of 100mV,

the accuracy is approximately ±7%. Using a lower cur-

rent-sense threshold results in less accuracy. The cur-

rent-sense resistor only dissipates power when the

synchronous rectifier is on.

For lower power dissipation, the MAX8734 uses the on-

resistance of the synchronous rectifier as the current-

sense element. Use the worst-case maximum value for

RDS(ON) from the MOSFET data sheet, and add some

margin for the rise in RDS(ON) with temperature. A good

general rule is to allow 0.5% additional resistance for

each °C of temperature rise. The current limit varies

with the on-resistance of the synchronous rectifier. The

reward for this uncertainty is robust, lossless overcur-

rent sensing. When combined with the undervoltage-

ON-TIME (tON)

ZERO-CROSSING

DETECTION

ISONIC

0

40µs (MAX)

INDUCTOR

CURRENT

Figure 6. Ultrasonic Current Waveforms

ILIMIT

ILOAD

0

TIME

-IPEAK

Figure 7. “Valley” Current-Limit Threshold Point

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