Rainbow Electronics MAX786 - Manual (Page 13)

MAX786
Dual-Output Power-Supply
Controller for Notebook Computers
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Table 1. Surface-Mount Components
(See Figure 1 for Standard Application Circuit.)
Internal VL and REF Supplies
An internal linear regulator produces the 5V used by the
internal control circuits. This regulator’s output is avail-
able on pin VL and can source 5mA for external loads.
Bypass VL to GND with 4.7µF. To save power, when the
+5V switch-mode supply is above 4.5V, the internal lin-
ear regulator is turned off and the high-efficiency +5V
switch-mode supply output is connected to VL.
The internal 3.3V bandgap reference (REF) is powered
by the internal 5V VL supply. It can furnish up to 5mA.
Bypass REF to GND with 0.22µF, plus 1µF/mA of load
current. The main switching outputs track the reference
voltage. Loading the reference will reduce the main
outputs slightly, according to the reference load-regula-
tion error.
Both the VL and REF outputs remain active, even when
the switching regulators are turned off, to supply mem-
ory keep-alive power (see
Shutdown Mode section).
These linear-regulator outputs can be directly connected
to the corresponding step-down regulator outputs (i.e.,
REF to +3.3V, VL to +5V) to keep the main supplies alive
in standby mode. However, to ensure start-up, standby
load currents must not exceed 5mA on each supply.
Fault Protection
The +3.3V and +5V PWM supplies and the compara-
tors are disabled when either of two faults is present:
VL < +4.0V or REF < +2.8V (85% of its nominal value).
__________________Design Procedure
Figure 1’s schematic and Table 2’s component list
show values suitable for a 3A, +5V supply and a 3A,
+3.3V supply. This circuit operates with input voltages
from 6.5V to 30V, and maintains high efficiency with
output currents between 5mA and 3A (see the
Typical
Operating Characteristics ). This circuit’s components
may be changed if the design guidelines described in
this section are used — but before beginning the
design, the following information should be firmly
established:
COMPONENT SPECIFICATION MANUFACTURER PART NO.
C1, C10 33µF, 35V tantalum capacitors
AVX TPSE226M035R0100
Sprague 595D336X0035R
C2 4.7µF, 6V tantalum capacitor
AVX TAJB475M016
Sprague 595D475X0016A
C3 1µF, 20V tantalum capacitor
AVX TAJA105M025
Sprague 595D105X0020A2B
C4, C5 0.1µF, 16V ceramic capacitors Murata-Erie GRM42-6X7R104K50V
C6 330µF, 10V tantalum capacitor Sprague 595D337X0010R
C7, C12 150µF, 10V tantalum capacitors Sprague 595D157X0010D
C8, C9 0.01µF, 16V ceramic capacitors Murata-Erie GRM42-6X7R103K50V
D2A, D2B 1N4148-type dual diodes Central Semiconductor CMPD2836
D1, D3 1N5819 SMT diodes Nihon EC10QS04
L1, L2 10µH, 2.65A inductors Sumida CDR125-100
N1–N4 N-channel MOSFETs (SO-8) Siliconix Si9410DY
R1, R2 0.025
Ω
, 1% (SMT) resistors IRC LR2010-01-R025-F
COMPANY
FACTORY FAX
USA PHONE
[COUNTRY CODE]
AVX [1] (803) 626-3123
(803) 946-0690
(800) 282-4975
Central Semiconductor [1] (516) 435-1824 (516) 435-1110
IRC [1] (512) 992-3377 (512) 992-7900
Murata-Erie [1] (814) 238-0490 (814) 237-1431
Nihon [81] 3-3494-7414 (805) 867-2555
Siliconix [1] (408) 970-3950 (408) 988-8000
Sprague [1] (603) 224-1430 (603) 224-1961
Sumida [81] 3-3607-5144 (847) 956-0666
Table 2. Component Suppliers