MAX17500AEVKIT+ Maxim Integrated Products, MAX17500AEVKIT+ Datasheet - Page 11

• Manufacturer Part Number: MAX17500AEVKIT+
• Description: Power Management Modules & Development Tools MAX17500A EVAL KIT MAX17500A EVAL KIT
• Manufacturer: Maxim Integrated Products

Specifications of MAX17500AEVKIT+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The devices’ soft-start feature allows the output voltage
to ramp up in a controlled manner, eliminating voltage
overshoot. The devices’ reference generator that is
internally connected to the error amplifier soft-starts to
achieve superior control of the output voltage under
heavy- and light-load conditions. Soft-start begins after
UVLO is deasserted (V
MAX17500, V
the voltage on UVLO/EN is above 1.23V). The voltage
applied to the noninverting node of the amplifier ramps
from 0 to 1.23V in 1984 NDRV switching cycles. Use the
following formula to calculate the soft-start time (t
where f
output. Figure 4 shows the soft-start regulated output of
a power supply using the MAX17500 during startup.
The NDRV output drives an external n-channel MOSFET.
The internal regulator output (V
9V, drives NDRV. For the universal input voltage range,
the MOSFET used must withstand the DC level of the
high-line input voltage plus the reflected voltage at the
primary of the transformer. Most applications that use the
discontinuous flyback topology require a MOSFET rated
at 600V. NDRV can source/sink in excess of 650mA/
1000mA peak current; therefore, select a MOSFET that
mA yields acceptable conduction and switching losses.
Use an external resistor at RT to program the devices’
internal oscillator frequency between 50kHz and 2.5MHz.
The MAX17499A/MAX17500A output switching frequen-
cy is one-half the programmed oscillator frequency with a
50% duty cycle. The MAX17499B/MAX17500B output
switching frequency is one-quarter of the programmed
oscillator frequency with a 75% duty cycle.
The MAX17499A/MAX17500A and MAX17499B/
MAX17500B have programmable output switching fre-
quencies from 25kHz to 625kHz and 12.5kHz to
625kHz, respectively. Use the following formulas to
determine the appropriate value of resistor R12 (see
Figure 1) needed to generate the desired output
switching frequency (f
R
R
12
12
NDRV
=
= =
10
2
10
4
n-Channel MOSFET Switch Driver
f
f
SW
SW
IN
is the switching frequency at the NDRV
10
10
Oscillator/Switching Frequency
is above 9.5V for the MAX17499, and
for the MAX
for the MAX
______________________________________________________________________________________
t
SW
SS
) at the NDRV output:
=
IN
f
Programmable Switching Frequency
1984
NDRV
17499
17499
Current-Mode PWM Controllers with
is above 21.6V for the
CC
), set to approximately
B MAX
A MAX
17500
17500
Soft-Start
B
SS
A
.
.
):
where R12 is the resistor connected from RT to GND
(see Figure 1).
Connect an RC network in parallel with R12 as shown in
Figure 1. The RC network should consist of a 100nF
capacitor, C6, (for stability) in series with resistor R15,
which serves to further minimize jitter. Use the following
formula to determine the value of R15:
For example, if R12 is 4kΩ, R15 becomes 707Ω.
The devices include an internal error amplifier to regu-
late the output voltage in the case of a nonisolated
power supply (see Figure 1). For the circuit in Figure 1,
calculate the output voltage using the following equation:
where V
is internally connected to a digital soft-start circuit that
gradually increases the reference voltage during start-
up applied to this input. This forces the output voltage
to come up in an orderly and well-defined manner
under all load conditions.
The error amplifier may also be used to regulate the ter-
tiary winding output, which implements a primary-side-
regulated, isolated power supply (see Figure 6). For the
Figure 4. Primary-Side Output Voltage Soft-Start During Initial
Startup for the Circuit in Figure 6
REF
= 1.23V. The amplifier’s noninverting input
V
R
OUT
15 88 9
=
=
⎛
⎝ ⎜
2ms/div
1
.
Internal Error Amplifier
+
100mA LOAD ON/V
100mA LOAD ON/V
×
R
R
(
13
14
R
12
MAX17499/MAX17500 fig04
⎞
⎠ ⎟
)
V
4
1
REF
OUT1
OUT2
V
2V/div
OUT
11