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MAX15004 Datasheet(PDF) 17 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) 17 Page - Maxim Integrated Products

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IO is the load current,
ΔVQ is the portion of the ripple due
to the capacitor discharge, and
ΔVESR is the contribution
due to the ESR of the capacitor. DMAX is the maximum
duty cycle at the minimum input voltage. Use a combina-
tion of low-ESR ceramic and high-value, low-cost alu-
minum capacitors for lower output ripple and noise.
Calculating Power Loss in Boost Converter
The MAX15004A/MAX15005A devices are available in
a thermally enhanced package and can dissipate up to
1.7W at +70°C ambient temperature. The total power
dissipation in the package must be limited so that the
junction temperature does not exceed its absolute max-
imum rating of +150°C at maximum ambient tempera-
ture; however, Maxim recommends operating the
junction at about +125°C for better reliability.
The average supply current (IDRIVE-GATE) required by
the switch driver is:
where Qg is total gate charge at 7.4V, a number avail-
able from MOSFET datasheet.
The supply current in the MAX15004A/B/MAX15005A/B
is dependent on the switching frequency. See the
Typical Operating Characteristics to find the supply
current ISUPPLY of the MAX15004A/B/MAX15005A/B at
a given operating frequency. The total power dissipa-
tion (PT) in the device due to supply current (ISUPPLY)
and the current required to drive the switch (IDRIVE-
GATE) is calculated using following equation.
MOSFET Selection in Boost Converter
The MAX15004A/B/MAX15005A/B drive a wide variety of
n-channel power MOSFETs. Since VCC limits the OUT
output peak gate-drive voltage to no more than 11V, a
12V (max) gate voltage-rated MOSFET can be used with-
out an additional clamp. Best performance, especially at
low-input voltages (5VIN), is achieved with low-threshold
n-channel MOSFETs that specify on-resistance with a
gate-source voltage (VGS) of 2.5V or less. When selecting
the MOSFET, key parameters can include:
1) Total gate charge (Qg).
2) Reverse-transfer capacitance or charge (CRSS).
3) On-resistance (RDS(ON)).
4) Maximum drain-to-source voltage (VDS(MAX)).
5) Maximum gate frequencies threshold voltage
(VTH(MAX)).
At high switching, dynamic characteristics (parameters 1
and 2 of the above list) that predict switching losses
have more impact on efficiency than RDS(ON), which pre-
dicts DC losses. Qg includes all capacitances associat-
ed with charging the gate. The VDS(MAX) of the selected
MOSFET must be greater than the maximum output volt-
age setting plus a diode drop. The 10V additional margin
is recommended for spikes at the MOSFET drain due to
the inductance in the rectifier diode and output capacitor
path. In addition, Qg helps predict the current needed to
drive the gate at the selected operating frequency when
the internal LDO is driving the MOSFET.
Slope Compensation in Boost Configuration
The MAX15004A/B/MAX15005A/B use an internal ramp
generator for slope compensation to stabilize the current
loop when operating at duty cycles above 50%. It is
advisable to add some slope compensation even at lower
than 50% duty cycle to improve the noise immunity. The
slope compensations should be optimized because too
much slope compensation can turn the converter into the
voltage-mode control. The amount of slope compensation
required depends on the downslope of the inductor cur-
rent when the main switch is off. The inductor downslope
depends on the input to output voltage differential of the
boost converter, inductor value, and the switching fre-
quency. Theoretically, the compensation slope should be
equal to 50% of the inductor downslope; however, a little
higher than 50% slope is advised.
Use the following equation to calculate the required
compensating slope (mc) for the boost converter:
The internal ramp signal resets at the beginning of
each cycle and slews at the rate programmed by the
external capacitor connected to SLOPE. 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.
C
mc mV s
SLOPE =
×
25 10 9
.
()
μ
mc
VV
R
L
mV s
OUT
IN
S
=
××
()
()
10
2
3
μ
PV
I
INMAX
SUPPLY
TDRIVE GATE
I
+
()
IQ
f
DRIVE GATE
g
OUT
ESR
V
I
C
ID
Vf
ESR
O
OUT
OMAX
QOUT
=
=
×
×
Δ
Δ
4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
______________________________________________________________________________________
17


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