MAX1631AEAI+T Maxim Integrated Products, MAX1631AEAI+T Datasheet - Page 14
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MAX1631AEAI+T
Manufacturer Part Number
MAX1631AEAI+T
Description
IC PWR-SUPP CTRLR NOTEBK 28-SSOP
Manufacturer
Maxim Integrated Products
Type
Step-Down (Buck)r
Datasheet
1.MAX1634CAI.pdf
(29 pages)
Specifications of MAX1631AEAI+T
Internal Switch(s)
No
Synchronous Rectifier
Yes
Number Of Outputs
2
Voltage - Output
3.3V, 5V, Adj
Current - Output
4A
Frequency - Switching
200kHz, 300kHz
Voltage - Input
4.2 ~ 30 V
Operating Temperature
-40°C ~ 85°C
Mounting Type
Surface Mount
Package / Case
28-SSOP
Power - Output
762mW
Output Voltage
2.5 V to 5.5 V, 3.3 V, 5 V
Input Voltage
4.2 V to 30 V
Mounting Style
SMD/SMT
Maximum Operating Temperature
+ 85 C
Minimum Operating Temperature
- 40 C
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
under all operating conditions, including Idle Mode.
The SECFB signal further controls the synchronous
switch timing in order to improve multiple-output cross-
regulation (see Secondary Feedback Regulation Loop
section).
An internal regulator produces the +5V supply (VL) that
powers the PWM controller, logic, reference, and other
blocks within the IC. This 5V low-dropout linear regula-
tor supplies up to 25mA for external loads, with a
reserve of 25mA for supplying gate-drive power.
Bypass VL to GND with 4.7µF.
Important: Ensure that VL does not exceed 6V.
Measure VL with the main output fully loaded. If it is
pumped above 5.5V, either excessive boost diode
capacitance or excessive ripple at V+ is the probable
cause. Use only small-signal diodes for the boost cir-
cuit (10mA to 100mA Schottky or 1N4148 are pre-
ferred), and bypass V+ to PGND with 4.7µF directly at
the package pins.
The 2.5V reference (REF) is accurate to ±2% over tem-
perature, making REF useful as a precision system ref-
erence. Bypass REF to GND with 1µF minimum. REF
can supply up to 5mA for external loads. (Bypass REF
with a minimum 1µF/mA reference load current.)
However, if extremely accurate specifications for both
the main output voltages and REF are essential, avoid
loading REF more than 100µA. Loading REF reduces
the main output voltage slightly, because of the refer-
ence load-regulation error.
When the 5V main output voltage is above 4.5V, an
internal P-channel MOSFET switch connects CSL5 to
VL, while simultaneously shutting down the VL linear
regulator. This action bootstraps the IC, powering the
internal circuitry from the output voltage, rather than
through a linear regulator from the battery.
Bootstrapping reduces power dissipation due to gate
charge and quiescent losses by providing that power
from a 90%-efficient switch-mode source, rather than
from a much less efficient linear regulator.
Gate-drive voltage for the high-side N-channel switches
is generated by a flying-capacitor boost circuit
(Figure 2). The capacitor between BST_ and LX_ is
alternately charged from the VL supply and placed par-
allel to the high-side MOSFET’s gate-source terminals.
On start-up, the synchronous rectifier (low-side
MOSFET) forces LX_ to 0V and charges the boost
capacitors to 5V. On the second half-cycle, the SMPS
Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
14
______________________________________________________________________________________
Boost High-Side Gate-Drive Supply
Internal VL and REF Supplies
(BST3 and BST5)
turns on the high-side MOSFET by closing an internal
switch between BST_ and DH_. This provides the nec-
essary enhancement voltage to turn on the high-side
switch, an action that “boosts” the 5V gate-drive signal
above the battery voltage.
Ringing at the high-side MOSFET gate (DH3 and DH5)
in discontinuous-conduction mode (light loads) is a nat-
ural operating condition. It is caused by residual ener-
gy in the tank circuit, formed by the inductor and stray
capacitance at the switching node, LX. The gate-drive
negative rail is referred to LX, so any ringing there is
directly coupled to the gate-drive output.
The current-limit circuit resets the main PWM latch and
turns off the high-side MOSFET switch whenever the
voltage difference between CSH and CSL exceeds
100mV. This limiting is effective for both current flow
directions, putting the threshold limit at ±100mV. The
tolerance on the positive current limit is ±20%, so the
external low-value sense resistor (R1) must be sized for
80mV/I
current to support the full load current, while compo-
nents must be designed to withstand continuous cur-
rent stresses of 120mV/R1.
For breadboarding or for very-high-current applications,
it may be useful to wire the current-sense inputs with a
twisted pair, rather than PC traces. (This twisted pair
needn’t be anything special; two pieces of wire-wrap
wire twisted together are sufficient.) This reduces the
possible noise picked up at CSH_ and CSL_, which can
cause unstable switching and reduced output current.
The CSL5 input also serves as the IC’s bootstrap sup-
ply input. Whenever V
connects CSL5 to VL.
The SYNC input controls the oscillator frequency. Low
selects 200kHz; high selects 300kHz. SYNC can also
be used to synchronize with an external 5V CMOS or
TTL clock generator. SYNC has a guaranteed 240kHz
to 350kHz capture range. A high-to-low transition on
SYNC initiates a new cycle.
300kHz operation optimizes the application circuit for
component size and cost. 200kHz operation provides
increased efficiency, lower dropout, and improved
load-transient response at low input-output voltage dif-
ferences (see Low-Voltage Operation section).
PEAK
Current-Limiting and Current-Sense
, where I
PEAK
CSL5
Oscillator Frequency and
Synchronization (SYNC)
is the required peak inductor
Inputs (CSH and CSL)
> 4.5V, an internal switch