MIC2196 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – 400kHz SO-8 Boost Control IC

MIC2196
The mode of operation (continuous or discontinuous), the
minimum input voltage, maximum output power and the
minimum value of the current limit threshold determine the
value of the current sense resistor. Discontinuous mode is
where all the energy in the inductor is delivered to the output
at each switching cycle. Continuous mode of operation
occurs when current always flows in the inductor, during both
the low-side MOSFET on and off times. The equations below
will help to determine the current sense resistor value for
each operating mode.
The critical value of output current in a boost converter is
calculated below. The operating mode is discontinuous if the
output current is below this value and is continuous if above
this value.
( ) _ICRIT=
V_IN2 × V_O − V_IN ×
2 × fs × L × V_O2
η
where:
η is the efficiency of the boost converter
V_IN is the minimum input voltage
L is the value of the boost inductor
F_S is the switching frequency
V_O is the output voltage
Maximum Peak Current in Discontinuous Mode:
The peak inductor current is:
( ) _IIND(pk)=
2 × I_O × V_O − η × V_IN
L × fs
where:
I_O is the maximum output current
V_O is the output voltage
V_IN is the minimum input voltage
L is the value of the boost inductor
f_S is the switching frequency
η is the efficiency of the boost converter
The maximum value of current sense resistor is:
R_SENSE=
V_SENSE
I_IND(pk)
where:
V is the minimum current sense threshold of the
CS pin.
Maximum Peak Current in Continuous Mode:
The peak inductor current is equal to the average inductor
current plus one half of the peak to peak inductor current.
The peak inductor current is:
I_IND(pk)=
I_IND(ave)
+
1
2
×
I_IND(pp)
( ) _IIND(pk)=
V_O × I_O
V_IN × η
+
V_L × V_O
2 × V_O
− V_IN × η
× fs × L
where:
Micrel
I_O is the maximum output current
V_O is the output voltage
V_IN is the minimum input voltage
L is the value of the boost inductor
f_S is the switching frequency
η is the efficiency of the boost converter
V_L is the voltage across the inductor
V_L may be approximated as V_IN for higher input voltage.
However, the voltage drop across the inductor winding resis-
tance and low-side MOSFET on-resistance must be ac-
counted for at the lower input voltages that the MIC2196
operates at:
( ) ^V_L⁼
V_IN
−
V_O × I_O
V_IN × η
×
R_WINDING
+ R_DSON
where:
R_WINDING is the winding resistance of the inductor
R_DSON is the on resistance of the low side switching
MOSFET
The maximum value of current sense resistor is:
R_SENSE=
V_SENSE
I_IND(pk)
where:
V_SENSE is the minimum current sense threshold
of the CS pin.
The current sense pin, CS, is noise sensitive due to the low
signal level. The current sense voltage measurement is
referenced to the signal ground pin of the MIC2196. The
current sense resistor ground should be located close to the
IC ground. Make sure there are no high currents flowing in this
trace. The PCB trace between the high side of the current
sense resistor and the CS pin should also be short and routed
close to the ground connection. The input to the internal
current sense amplifier has a 30ns dead time at the beginning
of each switching cycle. This dead time prevents leading
edge current spikes from prematurely terminating the switch-
ing cycle. A small RC filter between the current sense pin and
current sense resistor may help to attenuate larger switching
spikes or high frequency switching noise. Adding the filter
slows down the current sense signal, which has the effect of
slightly raising the overcurrent limit threshold.
MOSFET Gate Drive
The MIC2196 converter drives a low-side N-channel MOSFET.
The driver for the OUTN pin has a 2Ω typical source and sink
impedance. The VIN pin is the supply pin for the gate drive
circuit. The maximum supply voltage to the VIN pin is 14V.
MOSFET Selection
In a boost converter, the V_DS of the MOSFET is approxi-
mately equal to the output voltage. The maximum V_DS rating
of the MOSFET must be high enough to allow for ringing and
spikes in addition to the output voltage.
The VIN pin supplies the N-channel gate drive voltage. The
V_GS threshold voltage of the N-channel MOSFET must be
MIC2196
8
August 2004