MIC2185_05 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
Micrel, Inc.
( ) ^VL⁼
V_IN
− ^{VO × IO}
V_IN × η
×
R_WINDING
+ R_DS(ON)
where:
R_WINDING is the winding resistance of the inductor
R_DS(ON) 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 MIC2185. 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
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 MIC2185 synchronous boost converter drives both a
high side and low side MOSFET. The low side drive, OUTN,
drives an n-channel MOSFET. The high-side drive, OUTP,
is designed to switch a p-channel MOSFET (the p-channel
MOSFET doesn't require a bootstrap circuit which would
be needed to drive an n-channel MOSFET). The V_INP pin
must be connected to the output, which provides power to
drive the high and low side MOSFETs. In skip mode, the
high side MOSFET is disabled by forcing the OUTP pin to
be high (equal to V_OUT).
MOSFET Selection
In a boost converter, the V_DS of the MOSFET, Q1, is ap-
proximately equal to the output voltage. The maximum Vds
rating of the MOSFET must be high enough to allow for ring-
ing and spikes. The MIC2185 input voltage range is 2.9V to
14V. MOSFETs with 20V and 30V V_DS ratings are ideal for
use with this part.
The n-channel gate drive voltage is supplied by the OUTN
output. At startup in a boost converter, the output voltage
equals the input voltage. The V_GS threshold voltage of the
n-channel MOSFET must be low enough to operate at the
minimum input voltage to guarantee the boost converter will
start up. The p-channel MOSFET must have a minimum
threshold voltage equal to or lower than the output voltage.
Five volt threshold (logic level) MOSFETs are recommended
for the p-channel MOSFET. Ringing in the gate drive signal
may cause MOSFETs with lower gate thresholds to errone-
ously turn on.
There is a limit to the maximum amount of gate charge the
MIC2185 will drive. Higher gate charge will slow down the
turn-on and turn-off times of the MOSFETs. The MOSFET’s
must be able to completely turn on and off within the driver
non-overlap time or shoot-through will occur.
MOSFET gate charge is also limited by power dissipation in
the MIC2186. The power dissipated by the gate drive circuitry
is calculated below:
P_{GATE_DRIVE} =Q_GATE · V_INP · f_S
where: Q_GATE is the total gate charge of both of the external
n- and p-channel MOSFETs.
The graph in Figure 7 shows the total gate charge which can
be driven by the MIC2185 over the input voltage range, for
different values of switching frequency.
Frequency vs.
Maximum Gate Charge
140
120
400kHz
2 0 0 kH z
3 0 0 kH z
100
80
60
5 0 0 kH z
40
6 0 0 kH z
20
03
5
7
9 11 13
INPUT VOLTAGE (V)
Figure 7 - MIC2185 Frequency vs. Max. Gate Charge
External Schottky Diode
An external boost diode in parallel with the high side MOSFET
is used to keep the inductor current flow continuous during
the non-overlap time when both MOSFETs are turned off.
Although the average current through this diode is small,
the diode must be able to handle currents equal to the peak
inductor current. This peak current is calculated in the Cur-
rent Limit section of this specification
The reverse voltage requirement of the diode is:
V_{DIODE_RRM}= V_OUT
For the MIC2185, Schottky diodes with a 30V or 40V rating
are recommended. Schottky diodes with lower reverse volt-
age ratings have higher reverse leakage current which will
cause ringing and excessive power dissipation in the diode
and low side MOSFET.
The external Schottky diode is not necessary for circuit
operation since the high side MOSFET contains a parasitic
body diode. However, the body diode has a relatively slow
reverse recovery time and a relatively high forward voltage
drop. The lower forward voltage drop of the Schottky diode
both prevents the parasitic diode from turning on and im-
proves efficiency. The lack of a reverse recovery mechanism
in a Schottky diode causes less ringing than the MOSFET's
parasitic diode. Depending on the circuit components and
operating conditions, an external Schottky diode will improve
the converter efficiency by ¹/₂% to 1%.
October 2005
11
MIC2185