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MAX15004 Datasheet(PDF) 14 Page - Maxim Integrated Products

Part No. MAX15004
Description  4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers
Download  27 Pages
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX15004 Datasheet(HTML) 14 Page - Maxim Integrated Products

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4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
14
______________________________________________________________________________________
n-Channel MOSFET Driver
OUT drives the gate of an external n-channel MOSFET.
The driver is powered by the internal regulator (VCC),
internally set to approximately 7.4V. If an external voltage
higher than 7.4V is applied at VCC (up to 10V), it appears
as the peak gate drive voltage. The regulated VCC 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:
where VREF = 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.
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:
where CSLOPE is the external capacitor at SLOPE in
farads.
Current Limit
The current-sense resistor (RCS), connected between
the source of the MOSFET and ground, sets the current
limit. The CS input has a voltage trip level (VCS) 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 RS:
where IPRI is the peak current that flows through the
MOSFET at full load and minimum VIN.
R
V
I
S
CS
PRI
=
Slope compensation mV s
A
CSLOPE
()
.( )
μ=
×
25 10 9
V
R
R
V
OUT
A
B
REF
=+
⎝⎜
⎠⎟
1
Figure 3a. MAX15005 Maximum Duty Cycle vs. Output
Frequency.
MAX15005 MAXIMUM DUTY CYCLE
vs. OUTPUT FREQUENCY (fOUT)
OUTPUT FREQUENCY (kHz)
50
55
60
65
70
75
80
85
90
95
100
10
100
1000
CT = 220pF
CT = 1500pF
CT = 1000pF
CT = 560pF
CT = 2200pF
CT = 3300pF
CT = 100pF
Figure 3b. Oscillator Frequency vs. RT/CT
OSCILLATOR FREQUENCY (fOSC)
vs. RT/CT
RT (k
Ω)
1
10
100
1000
10
100
1000
CT = 220pF
CT = 1500pF
CT = 1000pF
CT = 560pF
CT = 2200pF
CT = 3300pF
CT = 100pF


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