MAX15004_11 Datasheet, PDF (17/27 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
ESR = ^ΔVESR
I_O
C _OUT
=
I_O × D_MAX
ΔV_Q × f_OUT
I_O is the load current, ΔV_Q is the portion of the ripple due
to the capacitor discharge, and ΔV_ESR is the contribution
due to the ESR of the capacitor. D_MAX 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 (I_DRIVE-GATE) required by
the switch driver is:
I_{DRIVE−GATE} = Q_g × f_OUT
where Q_g 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 I_SUPPLY of the MAX15004A/B/MAX15005A/B at
a given operating frequency. The total power dissipa-
tion (P_T) in the device due to supply current (I_SUPPLY)
and the current required to drive the switch (I_DRIVE-
GATE^{) is calculated using following equation.}
P_T = V_INMAX × (I_SUPPLY + I_{DRIVE−GATE})
MOSFET Selection in Boost Converter
The MAX15004A/B/MAX15005A/B drive a wide variety of
n-channel power MOSFETs. Since V_CC 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 (5V_IN), is achieved with low-threshold
n-channel MOSFETs that specify on-resistance with a
gate-source voltage (V_GS) of 2.5V or less. When selecting
the MOSFET, key parameters can include:
1) Total gate charge (Q_g).
2) Reverse-transfer capacitance or charge (C_RSS).
3) On-resistance (R_DS(ON)).
4) Maximum drain-to-source voltage (V_DS(MAX)).
5) Maximum gate frequencies threshold voltage
(V_TH(MAX)).
At high switching, dynamic characteristics (parameters 1
and 2 of the above list) that predict switching losses
have more impact on efficiency than R_DS(ON), which pre-
dicts DC losses. Q_g includes all capacitances associat-
ed with charging the gate. The V_DS(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, Q_g 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:
( ) mc = (VOUT − VIN) × RS × 10 −3 mV μs
2L
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:
C _SLOPE
=
2.5 × 10 ^−9
mc(mV μs)
where C_SLOPE is the external capacitor at SLOPE in
farads.
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