User guide
MAX8737
Detailed Description
The MAX8737 is a dual, low-dropout, external n-chan-
nel linear regulator controller for low-voltage notebook
computer power supplies. The linear regulator provides
a 0.5V to 2.5V (±5mV no-load) output for powering the
low-voltage supplies to desktop and notebook CPU
chipsets (V
CCP
and V
CC
_
MCH
). The regulator operates
from low input voltage, which also reduces the power
dissipation in the external n-channel MOSFET. The con-
troller powers the external MOSFET gate driver from the
standard 5V system supply.
The controller features independent enable inputs
(EN_), PGOOD outputs (PGOOD_), input undervoltage
lockout (UVLO), and output undervoltage protection
(UVP). The controller uses an adjustable reference
input (REFIN_) to set the nominal output voltage
(V
OUT
), which minimizes the cost and makes the stabili-
ty independent of the output voltage. An output UVP
timing depends on the magnitude of the voltage at
V
OUT
. The UVP detects and shuts down the LDO if the
output voltage drops below the nominal output voltage
(V
REFIN
). Each linear regulator features an adjustable
soft-start function, and generates a delayed PGOOD
signal that signals when the linear regulator is in regula-
tion. The MAX8737 uses an external resistor-divider in
series with the current-sense input (CS_), providing
foldback current-limit protection, and effectively reduc-
ing the short-circuit power dissipation. The MAX8737 is
available in a thin QFN package to reduce the thermal
impedance, and improve the thermal coupling between
the controller and the external MOSFETs.
REFIN Input
The low-cost linear regulator uses an adjustable refer-
ence input (REFIN_) to set the nominal output voltage,
which minimizes cost and simplifies the stability—the
stability calculation is independent of V
OUT
. The output
voltage accuracy depends on the accuracy of the
source generating the REFIN voltage. Multiple accurate
references are typically available elsewhere in the sys-
tem (such as the switching regulator providing the low-
voltage input supply). If lower output accuracy is
acceptable, divide down and filter another regulated
output voltage supply.
To set output voltage, select R2 = 100kΩ and select R1
using the following formula:
Soft-Start
When the LDO is activated, the respective DRV_ is
pulled up from GND with a typical soft-start current of
170µA. The soft-start current limits the output voltage
slew rate and also limits the initial current spike through
the external n-channel MOSFET. The slew rate is also
limited by the compensation capacitance used at the
DRV_ pin.
The maximum drain current during startup is the ratio of
C
OUT
to C
COMP
, multiplied by the soft-start current
I
SOFT
of 170µA (typ).
Enable and Power Good
The MAX8737 has independent enable control inputs
(EN1, EN2). Drive EN1 high to enable output 1. Drive
EN2 high to enable output 2. When EN_ is driven low,
the corresponding DRV_ and PGOOD_ pins are pulled
to GND, and the output is discharged through a 10Ω
switch.
There are two independent PGOOD_ outputs indicating
the supply status. PGOOD_ is pulled high 2ms after the
controller is enabled (EN_ is pulled high and V
CC
exceeds its UVLO threshold), and the output is in regu-
lation. If either output is out of regulation, the respective
PGOOD_ goes low immediately. The MAX8737 pulls
PGOOD_ low if the output voltage drops below the
lower trip threshold of -12% (typ) or when V
CC
is in
UVLO or when EN_ is pulled low.
Soft-Stop
The MAX8737 enables a soft-stop function that dis-
charges the output through an internal 10Ω switch
when EN_ is driven low or V
CC
is in UVLO. The dis-
charge time of the output depends on the output
capacitance, output load, and the exact resistance of
the internal discharge switch. To slow down the dis-
charge rate, add resistance in series with the OUT_ pin.
5.0V Bias Supply (V
CC
)
The linear regulator operates with very low input volt-
ages. V
IN
may be as low as 1.2V, so a secondary 5V
supply is required to provide sufficient bias to the gate
drivers. Locally decouple the V
CC
input with 1µF or
greater of ceramic capacitance.
Current Limit
The MAX8737 features a current limit that monitors the
voltage across the current-sense resistor, which limits
V
CS
_ - V
OUT
_ to 10mV (typ). However, in case of a
short-circuit condition, the power dissipation across the
external FET will be extremely high. To protect the
external FET, the MAX8737 uses an external resistive
divider (see Figure 1) to fold back the current limit,
reducing the overall power dissipation. The foldback
R
V
V
R
REF
REFIN
112=−
_
Dual, Low-Voltage Linear Regulator Controllers
with External MOSFETs
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