Max5099, Applications information – Rainbow Electronics MAX5099 User Manual
Page 17
MAX5099
Thermal-Overload Protection
During continuous short circuit or overload at the output,
the power dissipation in the IC can exceed its limit. The
MAX5099 provides thermal shutdown protection with
temperature hysteresis. Internal thermal shutdown is
provided to avoid irreversible damage to the device.
When the die temperature exceeds +165°C (typ), an on-
chip thermal sensor shuts down the device, forcing the
internal switches to turn off, allowing the IC to cool. The
thermal sensor turns the part on again with soft-start
after the junction temperature cools by +20°C. During
thermal shutdown, both regulators shut down, PGOOD_
goes low, and soft-start resets. The internal 20V zener
clamp from IN_HIGH to SGND is not turned off during
thermal shutdown because this clamping action must
always be active.
Applications Information
Setting the Switching Frequency
The controller generates the clock signal by dividing
down the internal oscillator f
OSC
or the SYNC input sig-
nal when driven by an external oscillator. The switching
frequency equals half the internal oscillator frequency
(f
SW
= f
OSC
/2). The internal oscillator frequency is set
by a resistor (R
OSC
) connected from OSC to SGND. To
find R
OSC
for each converter switching frequency f
SW
,
use the formulas:
A rising clock edge on SYNC is interpreted as a syn-
chronization input. If the SYNC signal is lost, the inter-
nal oscillator takes control of the switching rate,
returning the switching frequency to that set by R
OSC
.
When an external synchronization signal is used, R
OSC
must be selected such that f
SW
= 1/2 f
SYNC
.
Buck Converter
Effective Input Voltage Range
Although the MAX5099 converter operates from input
supplies ranging from 5.2V to 19V, the input voltage
range can be effectively limited by the MAX5099 duty-
cycle limitations for a given output voltage. The maximum
input voltage is limited by the minimum on-time
(t
ON(MIN)
):
where t
ON(MIN)
is 100ns. The minimum input voltage is
limited by the maximum duty cycle (D
MAX
= 0.92):
where V
DROP1
is the total parasitic voltage drops in the
inductor discharge path, which includes the forward
voltage drop (V
DS
) of the low-side n-channel MOSFET,
the series resistance of the inductor, and the PCB resis-
tance. V
DROP2
is the total resistance in the charging
path that includes the on-resistance of the high-side
switch, the series resistance of the inductor, and the
PCB resistance.
Setting the Output Voltage
For 0.8V or greater output voltages, connect a voltage-
divider from OUT_ to FB_ to SGND (Figure 2). Select
RB (FB_ to SGND resistor) to between 1kΩ and 20kΩ.
Calculate R
A
(OUT_ to FB_ resistor) with the following
equation:
where V
FB_
= 0.8V (see the
Electrical Characteristics
table).
For output voltages below 0.8V, set the MAX5099 out-
put voltage by connecting a voltage-divider from OUT_
to FB_ to BYPASS (Figure 2). Select RC (FB_ to
BYPASS resistor) higher than a 50kΩ range. Calculate
R
A
with the following equation:
where V
FB_
= 0.8V, V
BYPASS
= 2V (see the
Electrical
Characteristics
table), and V
OUT_
can range from 0V to
V
FB_
.
R
R
V
V
V
V
A
C
FB
OUT
BYPASS
FB
=
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
−
−
_
_
_
R
R
V
V
A
B
OUT
FB
=
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
−
_
_
1
V
V
V
D
V
V
IN MIN
OUT
DROP
MAX
DROP
DROP
(
)
=
+
⎡
⎣
⎢
⎤
⎦
⎥+
−
1
2
1
1
V
V
t
f
IN MAX
OUT
ON MIN
SW
(
)
(
)
≤
×
R
k
f
MHz
f
MHz
R
k
f
MHz
f
MHz
OSC
SW
SW
OSC
SW
SW
Ω
Ω
( )
=
(
)
≥
(
)
( )
=
(
)
<
(
)
10 721
1 25
12 184
1 25
0 920
0 973
.
.
.
.
.
.
Dual, 2.2MHz, Automotive Synchronous Buck
Converter with 80V Load-Dump Protection
______________________________________________________________________________________
17