MAX8734 Maxim Integrated Products, MAX8734 Datasheet - Page 20

• Manufacturer Part Number: MAX8734
• Description: (MAX8732 - MAX8734) Main Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
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-
Figure 6. Ultrasonic Current Waveforms
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______________________________________________________________________________________
0
Reference and Linear Regulators
I
SONIC
40µs (MAX)
(REF, LDO5, and LDO3)
ZERO-CROSSING
DETECTION
ON-TIME (t
ON
)
INDUCTOR
CURRENT
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.
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
R
margin for the rise in R
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-
Figure 7. “Valley” Current-Limit Threshold Point
DS(ON)
from the MOSFET data sheet, and add some
0
Current-Limit Circuit (ILIM_)
DS(ON)
TIME
with temperature. A good
-I
PEAK
I
I
LOAD
LIMIT