MAX15004_10 Datasheet, PDF (14/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
MAX15005 MAXIMUM DUTY CYCLE
vs. OUTPUT FREQUENCY (f_OUT)
100
CT = 100pF
95
90
85
80
75 ^{CT = 3300pF}
70 _{CT = 2200pF}
65
CT = 1500pF
60
55
CT = 1000pF ^{CT = 560pF}
CT = 220pF
50
10
100
1000
OUTPUT FREQUENCY (kHz)
Figure 3a. MAX15005 Maximum Duty Cycle vs. Output
Frequency.
n-Channel MOSFET Driver
OUT drives the gate of an external n-channel MOSFET.
The driver is powered by the internal regulator (V_CC),
internally set to approximately 7.4V. If an external voltage
higher than 7.4V is applied at V_CC (up to 10V), it appears
as the peak gate drive voltage. The regulated V_CC volt-
age keeps the OUT voltage below the maximum gate
voltage rating of the external MOSFET. OUT can source
750mA and sink 1000mA peak current. The average cur-
rent sourced by OUT depends on the switching frequen-
cy and total gate charge of the external MOSFET.
Error Amplifier
The MAX15004A/B/MAX15005A/B include an internal
error amplifier. The noninverting input of the error
amplifier is connected to the internal 1.228V reference
and feedback is provided at the inverting input. High
100dB open-loop gain and 1.6MHz unity-gain band-
width allow good closed-loop bandwidth and transient
response. Moreover, the source and sink current capa-
bility of 2mA provides fast error correction during the
output load transient. For Figure 5, calculate the power-
supply output voltage using the following equation:
V_OUT
=
⎛⎝⎜₁₊
R_A
R_B
⎞
⎠⎟
V_REF
where V_REF = 1.228V. The amplifier’s noninverting input
is internally connected to a soft-start circuit that gradu-
ally increases the reference voltage during startup. This
forces the output voltage to come up in an orderly and
well-defined manner under all load conditions.
OSCILLATOR FREQUENCY (f_OSC)
vs. RT/CT
1000
CT = 100pF
CT = 220pF
CT = 560pF
100
CT = 1500pF
CT = 2200pF
CT = 1000pF
CT = 3300pF
10
1
10
100
1000
RT (kΩ)
Figure 3b. Oscillator Frequency vs. RT/CT
Slope Compensation
The MAX15004A/B/MAX15005A/B use an internal ramp
generator for slope compensation. The internal ramp
signal resets at the beginning of each cycle and slews
at the rate programmed by the external capacitor con-
nected to SLOPE. The amount of slope compensation
needed depends on the downslope of the current
waveform. Adjust the MAX15004A/B/MAX15005A/B
slew rate up to 110mV/µs using the following equation:
Slope compensation (mV μs) = ^{2.5 × 10−9(A)}
C_SLOPE
where C_SLOPE is the external capacitor at SLOPE in
farads.
Current Limit
The current-sense resistor (R_CS), connected between
the source of the MOSFET and ground, sets the current
limit. The CS input has a voltage trip level (V_CS) of
305mV. The current-sense threshold has 5% accuracy.
Set the current-limit threshold 20% higher than the peak
switch current at the rated output power and minimum
input voltage. Use the following equation to calculate
the value of R_S:
R_S
=
V_CS
I_PRI
where I_PRI is the peak current that flows through the
MOSFET at full load and minimum V_IN.
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