MAX15034 Datasheet, PDF (20/25 Pages) Maxim Integrated Products – Configurable, Single-/Dual-Output, Synchronous Buck Controller for High-Current Applications

Configurable, Single-/Dual-Output, Synchronous
Buck Controller for High-Current Applications
Table 2. Low-Side MOSFET Losses
LOSS
DESCRIPTION
SEGMENT LOSSES
Conduction Loss ^{Losses associated with MOSFET on-time, IRMS}
is a function of load current and duty cycle.
Gate Drive Loss
Losses associated with charging and
discharging the gate of the MOSFET every
cycle. There is no Q_GD charging involved in this
MOSFET due to the zero-voltage turn-on. The
charge involved is (Q_G - Q_GD).
P_CONDUCTION = I_RMS2 × R_DS(ON)
where I_RMS ≈
V_IN − V_OUT
V_IN
× I_LOAD
( ) ^P_GATEDRIVE ^{= V}_DD ^{× Q}_G ^{− Q}_GD ^{× f}_SW
Note: The gate drive losses are distributed between the drivers and the MOSFETs in the ratio of the gate driver’s resistance and the
MOSFET’s internal gate resistance.
Keep the maximum output-voltage deviation less than
or equal to the adaptive voltage-positioning window
(ΔV_OUT). During a load step, assume a 50% contribu-
tion each from the output capacitance discharge and
the voltage drop across the ESR (ΔV_OUT = ΔV_{ESR_OUT}
+ ΔV_{Q_OUT}). Use the following equations to calculate
the required ESR and capacitance value:
R_{ESR _}
OUT
=
ΔV_{ESR_ OUT}
I_{LOAD_ STEP}
C_OUT
=
I_{LOAD_ STEP} × t_RESPONSE
ΔV_{Q_ OUT}
where I_{LOAD_STEP} is the step in load current and
t_RESPONSE is the response time of the controller.
Controller response time depends on the control-loop
bandwidth. C_OUT is C6 and C7 in Figure 6.
Current Limit
The MAX15034 incorporates two forward current-limit
protection mechanisms, average current limit and hic-
cup fault current limit, which accurately limit the output
current per phase. The average current-mode control
technique of the MAX15034 accurately limits the maxi-
mum average output current per phase. The
MAX15034 senses the voltage across either a sense
resistor or can implement lossless inductor sense,
sensing the voltage across the parasitic resistance of
the inductor (DCR). Use either mechanism to limit the
maximum inductor current.
The minimum average voltage, at which the voltage
across the current-sense resistor is clamped, is either
internally set to 20.4mV or is controlled by the voltage
at AVGLIMIT. The AVGLIMIT ground threshold of
550mV (typ) is the threshold above which the control of
the average current-limit voltage is transferred from the
internal 20.4mV (min) reference to the externally set
V_AVGLIMIT. For using the internal average current-limit
value, short AVGLIMIT to AGND. The minimum (inter-
nally set) average current limit is set at:
I_LIMIT(MIN)
=
20.4mV
R_SENSE
For example, the current-sense resistor:
R_SENSE
=
20.4mV
10A
=
2.04mΩ
for a maximum output current limit of 10A. A standard
value is 2mΩ. Also, adjust the value of the current-
sense resistor to compensate for parasitics associated
with the PCB. Select a noninductive resistor with an
appropriate wattage rating.
The implementation is shown in Figure 8.
When sensing directly across the inductor, connect an
RC circuit directly across the shunt or inductor (see
Figure 9).
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