MAX8650 Datasheet, PDF (20/25 Pages) Maxim Integrated Products – 4.5V to 28V Input Current-Mode Step-Down Controller with Adjustable Frequency

4.5V to 28V Input Current-Mode Step-Down
Controller with Adjustable Frequency
P_LSCC
=
⎛
⎜^1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
× I^2LOAD
×
R_DS(ON)
Use R_DS(ON) at T_J(MAX):
P_LSDC = 2 × I_LOAD × V_F × t_DT × f_S
where V_F is the body-diode forward-voltage drop, t_DT is
the dead time between high-side and low-side switching
transitions (30ns typ), and f_S is the switching frequency.
The high-side MOSFET operates as a duty-cycle con-
trol switch and has the following major losses: the
channel-conduction loss (P_HSCC), the VL overlapping
switching loss (P_HSSW), and the drive loss (P_HSDR).
The high-side MOSFET does not have body-diode con-
duction loss, unless the converter is sinking current,
when the loss due to body-diode conduction is calcu-
lated as P_HSDC = 2 x I_LOAD x V_F x t_DT x f_S:
P_HSCC
=
V_OUT
V_IN
× I^2LOAD
× R_DS(ON)
Use R_DS(ON) at T_J(MAX):
P_HSSW = V_IN × I_LOAD × ^{QGS + QGD} × f_S
I_GATE
where I_GATE is the average DH driver output-current
capability determined by:
I_GATE ≅
0.5 × V_VL
R_DS(ON)(DR) + R_GATE
where R_DS(ON)(DR) is the high-side MOSFET driver’s
on-resistance (1.5Ω typ) and R_GATE is the internal gate
resistance of the MOSFET (~2Ω):
P_HSDR
=
Q_G
×
V_GS
×
f_S
×
R_GATE
R_GATE
+ R_DS(ON)(DR)
where V_GS ≈ V_VL.
In addition to the losses above, allow approximately
20% more for additional losses due to MOSFET output
capacitances and low-side MOSFET body-diode
reverse-recovery charge dissipated in the high-side
MOSFET, but is not well defined in the MOSFET data
sheet. Refer to the MOSFET data sheet for thermal-
resistance specifications to calculate the PC board
area needed to maintain the desired maximum operat-
ing junction temperature with the above calculated
power dissipations.
To reduce EMI caused by switching noise, add a 0.1µF
ceramic capacitor from the high-side switch drain to
the low-side switch source or add resistors in series
with DH and DL to slow down the switching transitions.
However, adding series resistors increases the power
dissipation of the MOSFET, so ensure this does not
overheat the MOSFET.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple-current
requirement (I_RMS) imposed by the switching currents
defined by the following equation:
( ) ^ILOAD ^VOUT ^{× V}IN ^{− V}OUT
I_RMS =
V_IN
I_RMS has a maximum value when the input voltage
equals twice the output voltage (V_IN = 2 x V_OUT), so
I_RMS(MAX) = I_LOAD / 2. Ceramic capacitors are recom-
mended due to their low ESR and ESL at high frequen-
cy with relatively low cost. Choose a capacitor that
exhibits less than 10°C temperature rise at the maxi-
mum operating RMS current for optimum long-term reli-
ability. Ceramic capacitors with X5R or better
temperature characteristics are recommended.
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the equivalent series
resistance (ESR), the equivalent series inductance
(ESL), and the voltage-rating requirements. These
parameters affect the overall stability, output voltage
ripple, and transient response. The output ripple has
three components: variations in the charge stored in
the output capacitor, the voltage drop across the
capacitor’s ESR and ESL caused by the current into
and out of the capacitor. The maximum output voltage
ripple is estimated as follows:
V_RIPPLE = V_RIPPLE(ESR) + V_RIPPLE(C) + V_RIPPLE(ESL)
The output voltage ripple as a consequence of the
ESR, ESL, and output capacitance is:
V_RIPPLE(ESR) = I_P−P × ESR
V_RIPPLE(ESL)
=
L
V_IN
+ ESL
×
ESL
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