MAX1533AETJ+ Maxim Integrated Products, MAX1533AETJ+ Datasheet - Page 29

• Manufacturer Part Number: MAX1533AETJ+
• Description: IC POWER SUPPLY CONTROLER 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX1533AETJ+

• Applications: Power Supply Controller
• Voltage - Input: 4.5 ~ 26 V
• Current - Supply: 15µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The transformer turns ratio (N) is determined by:
where V
ondary voltage, V
secondary rectifier, V
the main output voltage, and V
age drop across the synchronous-rectifier MOSFET.
The transformer secondary return is often connected to
the main output voltage instead of ground to reduce
the necessary turns ratio. In this case, subtract V
from the secondary voltage (V
former turns-ratio equation above. The secondary
diode in coupled-inductor applications must withstand
flyback voltages greater than 60V. Common silicon rec-
tifiers, such as the 1N4001, are also prohibited
because they are too slow. Fast silicon rectifiers such
as the MURS120 are the only choice. The flyback volt-
age across the rectifier is related to the V
ference, according to the transformer turns ratio:
where N is the transformer turns ratio (secondary wind-
ings/primary windings), and V
ondary DC output voltage. If the secondary winding is
returned to V
from V
reverse-breakdown voltage rating must also accommo-
date any ringing due to leakage inductance. The
diode’s current rating should be at least twice the DC
load current on the secondary output.
The inductor ripple current also impacts transient-
response performance, especially at low V
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The total output voltage sag is the sum of the voltage
sag while the inductor is ramping up, and the voltage
sag before the next pulse can occur.
High-Efficiency, 5x Output, Main Power-Supply
FLYBACK
V
SAG
SEC
V
N
FLYBACK
=
=
is the minimum required rectified sec-
OUT5
V
OUT
2
in the equation above. The diode’s
C
FWD
______________________________________________________________________________________
Δ
OUT
5
= V
instead of ground, subtract V
I
V
LOAD MAX
OUT5(MIN)
SEC
+
L
SEC
is the forward drop across the
(
V
(
V
RECT
Δ
(
IN
+
C
I
+ (V
LOAD MAX
Controllers for Notebook Computers
OUT
V
×
Transient Response
SEC
SEC
FWD
)
RECT
IN
+
is the minimum value of
D
(
(
T
V
MAX
- V
- V
SENSE
is the maximum sec-
-
is the on-state volt-
OUT5
OUT5
)
Δ
)
-
T
2
)
V
IN
) x N
OUT
) in the trans-
IN
- V
- V
)
OUT5
OUT
+
OUT5
OUT5
dif-
dif-
where D
Electrical Characteristics table), T is the switching period
(1/f
mode, or L x 0.2 x I
The amount of overshoot during a full-load to no-load
transient due to stored inductor energy can be calculat-
ed as:
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The
peak inductor current occurs at I
the ripple current; therefore:
where I
voltage divided by the current-sense resistance
(R
limit threshold is 70mV.
Connect ILIM_ to V
threshold. In adjustable mode, the current-limit thresh-
old is precisely 1/10th the voltage seen at ILIM_. For an
adjustable threshold, connect a resistive divider from
REF to analog ground (GND) with ILIM_ connected to
the center tap. The external 500mV to 2V adjustment
range corresponds to a 50mV to 200mV current-limit
threshold. When adjusting the current limit, use 1% tol-
erance resistors and a divider current of approximately
10µA to prevent significant inaccuracy in the current-
limit tolerance.
The current-sense method (Figure 9) and magnitude
determine the achievable current-limit accuracy and
power loss. Typically, higher current-sense limits pro-
vide tighter accuracy, but also dissipate more power.
Most applications employ a current-limit threshold
(V
be determined by:
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure
LIMIT
SENSE
OSC
) of 50mV to 100mV, so the sense resistor can
LIMIT
), and ΔT equals V
). For the default setting, the minimum current-
MAX
I
LIMIT
equals the minimum current-limit threshold
V
SOAR
is the maximum duty factor (see the
>
R
I
LOAD MAX
SENSE
MAX
=
CC
/(V
Setting the Current Limit
(
(
Δ
2
for the default current-limit
= V
IN
I
LOAD MAX
C
OUT
)
- V
OUT
LIMIT
+
OUT
(
⎛
⎜
⎝
/V
Δ
V
/I
IN
I
) when in skip mode.
INDUCTOR
OUT
LOAD(MAX)
LIM
)
x T when in PWM
)
2
2
L
⎞
⎟
⎠
plus half
29