MIC2186 Datasheet, PDF (11/16 Pages) Micrel Semiconductor – Low Voltage PWM Control IC

MIC2186
R_SENSE=
V_SENSE
I_IND(pk)
where:
Vsense is the minimum current sense threshold
of the CSH 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:
Io is the maximum output current
Vo is the output voltage
Vin is the minimum input voltage
L is the value of the boost inductor
fs is the switching frequency
η is the efficiency of the boost converter
VL is the voltage across the inductor
VL may be approximated as Vin 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 MIC2186
operates at.
( ) ^V_L^{= V}_IN ^−
V_O × I_O
V_IN × η
×
R_WINDING + R_DSON
where:
Rwinding is the winding resistance of the inductor
Rdson 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 CSH pin
The current sense pin, CSH, is noise sensitive due to the low
signal level. The current sense voltage measurement is
referenced to the signal ground pin of the MIC2186. 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 CHS 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
Micrel
switching 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 MIC2186 converter drives a low side N-channel MOSFET.
The driver for the OutN pin has a 1.6Ω typical source and sink
impedance. The VinP pin is the supply pin for the gate drive
circuit. It typically connected to the output. The maximum
supply voltage to the VinP pin is 14V. If the output voltage is
greater than 14V or if it is desired to drive the MOSFET with
a voltage less than Vout, the VinP pin can be connected to the
input or to an separate supply voltage.
MOSFET Selection
In a boost converter, the Vds of the MOSFET is approxi-
mately equal to the output voltage. The maximum Vds rating
of the MOSFET must be high enough to allow for ringing and
spikes in addition to the output voltage.
The VinP pin supplies the N-channel gate drive voltage. The
Vgs threshold voltage of the N-channel MOSFET must be low
enough to operate at the minimum VinP voltage to guarantee
the boost converter will start up.
The maximum amout of MOSFET gate charge that can be
driven is limited by the power dissipation in the MIC2186. The
power dissipated by the gate drive circuitry is calculated
below:
P_gate_drive=Q_gate * VinP * fs
where:
Q_gate is the total gate charge of the external
MOSFET
The graph in Figure 7 shows the total gate charge which can
be driven by the MIC2186 over the input voltage range, for
different values of switching frequency. Higher gate charge
will slow down the turn-on and turn-off times of the MOSFET,
which increases switching losses.
Power Dissipation
vs. Frequency
220
200
100kHz
180
200kHz
160
140 _500kHz
400kHz
120
100 600kHz
80
60
40
20
00 _{2 4 6 8 10 12 14}
V_INP (V)
Figure 7 - MIC2186 freq vs pdiss
External Schottky Diode
In a boost converter topology, the boost diode, D1 must be
rated to handle the peak and average current. The average
current through the diode is equal to the average output
current of the boost converter. The peak current is calculated
in the current limit section of this specification.
July 2002
11
MIC2186