MAX1999EEI-T Maxim Integrated Products, MAX1999EEI-T Datasheet - Page 20

• Manufacturer Part Number: MAX1999EEI-T
• Description: DC/DC Switching Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1999EEI-T

• Number Of Outputs: 1
• Output Voltage: 3.3 V, 2 V to 5.5 V
• Output Current: 0.95 A
• Input Voltage: 6 V to 24 V
• Mounting Style: SMD/SMT
• Package / Case: QSOP-28
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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 ref-
erence. Bypass REF to GND with a 0.22µF minimum
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
supplies up to 100mA for external loads. Bypass LDO5
and LDO3 with a minimum of 4.7µF load, use an addi-
tional 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 regu-
lator. These actions bootstrap the device, powering the
internal circuitry and external loads from the output SMPS
voltages, rather than through linear regulators from the
battery. 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.
Figure 6. “Valley” Current-Limit Threshold Point
20
______________________________________________________________________________________
Reference and Linear Regulators
0
TIME
(REF, LDO5, and LDO3)
-I
PEAK
I
I
LOAD
LIMIT
The current-limit circuit employs a “valley” current-sens-
ing algorithm. The MAX1999 uses the on-resistance of
the synchronous rectifier, while the MAX1777/MAX19777
uses 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_
(MAX1777/MAX1977) / LX_ (MAX1999) is above the cur-
rent-limit threshold, the PWM is not allowed to initiate a
new cycle (Figure 6). 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 MAX1777/MAX1977, 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 MAX1999 uses the on-
resistance of the synchronous rectifier as the current-
sense element (Figure 7). Use the worst-case maximum
value for R
add some margin for the rise in R
ture. 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, loss-
less overcurrent sensing. When combined with the
undervoltage protection circuit, this current-limit
method is effective in almost every circumstance.
Figure 7. Current Sensing Using R
Rectifier
DS(ON)
MAX1999
from the MOSFET data sheet, and
Current Limit Circuit (ILIM_)
DH_
DL_
LX_
DS(ON)
V+
DS(ON)
of Synchronous
with tempera-
OUT_