MAX8741EAI+T Maxim Integrated, MAX8741EAI+T Datasheet - Page 20

• Manufacturer Part Number: MAX8741EAI+T
• Description: Current & Power Monitors & Regulators 500kHz Multi-Out Pwr Supply Controller
• Manufacturer: Maxim Integrated

Specifications of MAX8741EAI+T

• Rohs: yes
• Product: Power Monitors
• Supply Voltage - Max: 30 V
• Supply Voltage - Min: 4.2 V
• Input Voltage Range: 4.2 V to 30 V

500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
voltage. A linear postregulator may still be needed to
meet strict output-accuracy specifications.
The MAX8742 has a V
13.5V, set by an internal resistor-divider. The MAX8741
has an adjustable secondary-output voltage set by an
external resistor-divider on SECFB (Figure 5). Ordinarily,
the secondary regulation point is set 5% to 10% below
the voltage normally produced by the flyback effect. For
example, if the output voltage as determined by turns
ratio is 15V, set the feedback resistor ratio to produce
13.5V. Otherwise, the SECFB one-shot might be triggered
unintentionally, unnecessarily increasing supply current
and output noise.
The MAX8742 includes a 12V linear-regulator output
capable of delivering 120mA of output current.
Typically, greater current is available at the expense of
output accuracy. If an accurate output of more than
120mA is needed, an external pass transistor can be
added. The circuit in Figure 6 delivers more than
200mA. Total output current is constrained by the V+
input voltage and the transformer primary load (see the
Maximum V
in the Typical Operating Characteristics).
Figure 5. Adjusting the Secondary Output Voltage with SECFB
20
12V Linear-Regulator Output (MAX8742)
1-SHOT
______________________________________________________________________________________
TRIG
MAX8741
+V
TRIP
DD
2.5V REF
= V
SECFB
DH_
Output Current vs. Input Voltage graphs
DL_
REF
(
1 + –––
R1
R2
V+
DD
)
pin that regulates at a fixed
WHERE V
REF
(NOMINAL) = 2.5V
R2
R1
POSITIVE
SECONDARY
OUTPUT
MAIN
OUTPUT
The three predesigned 3V/5V standard application cir-
cuits (Figure 1 and Table 1) contain ready-to-use solu-
tions for common application needs. Also, one
standard flyback transformer circuit supports the
12OUT linear regulator in the Applications Information
section. Use the following design procedure to optimize
these basic schematics for different voltage or current
requirements. Before beginning a design, however,
firmly establish the following:
• Maximum Input (Battery) Voltage, V
• Minimum Input (Battery) Voltage, V
Figure 6. Increased 12V Linear-Regulator Output Current
value should include the worst-case conditions,
such as no-load operation when a battery charger or
AC adapter is connected but no battery is installed.
V
should be taken at full load under the lowest battery
conditions. If V
nal circuit to externally hold V
age- lockout threshold. If the minimum input-output
difference is less than 1.5V, the filter capacitance
required to maintain good AC load regulation increas-
es (see the Low-Voltage Operation section).
IN(MAX)
MAX8742
must not exceed 30V.
12OUT
DH_
V
DL_
DD
IN(MIN)
V+
0.1μF
0.1μF
is less than 4.2V, use an exter-
Design Procedure
0.1μF
10Ω
L
above the V
2N3906
IN(MAX)
IN(MIN)
10μF
L
2.2μF
12V OUTPUT
200mA
MAIN
OUTPUT
V
undervolt-
DD
OUTPUT
. This
.This