MIC2159 Datasheet, PDF (9/17 Pages) Micrel Semiconductor – SYNCHRONOUS-itty™ Step-Down Converter IC

Micrel
Application information
MOSFET Selection
The MIC2159 controller works from input voltages of 3V
to 14.5V and has an internal 5V regulator to provide
power to turn the external N-Channel power MOSFETs
for high- and low-side switches. For applications where
V_IN < 5V, the internal V_DD regulator operates in dropout
mode, and it is necessary that the power MOSFETs
used are sub-logic level and are in full conduction mode
for V_GS of 2.5V. For applications when VIN > 5V; logic-
level MOSFETs, whose operation is specified at VGS =
4.5V must be used. For the lower (<5v) applications, the
V_DD supply can be connected directly to Vin to help
increase the driver voltage to the MOSFET.
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in
junction temperature will increase the channel resistance
of the MOSFET by 50% to 75% of the resistance
specified at 25°C. This change in resistance must be
accounted for when calculating MOSFET power
dissipation and in calculating the value of current-sense
(CS) resistor. Total gate charge is the charge required to
turn the MOSFET on and off under specified operating
conditions (V_DS and V_GS). The gate charge is supplied by
the MIC2159 gate-drive circuit. At 400kHz switching
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2159.
At low output load, this power dissipation is noticeable
as a reduction in efficiency. The average current
required to drive the high-side MOSFET is:
I_G[high-side](avg)=Q_G x F_S
Where:
I_G[high-side](avg)=Average high-side MOSFET gate
current
Q_G = total gate charge for the high-side MOSFET
taken from manufacturer’s data sheet for VGS =
5V.
F_S = Switching Frequency (400kHz)
The low-side MOSFET is turned on and off at VDS = 0
because the freewheeling diode is conducting during this
time. The switching loss for the low-side MOSFET is
usually negligible. Also, the gate-drive current for the
low-side MOSFET is more accurately calculated using
C_ISS at V_DS = 0 instead of gate charge.
For the low-side MOSFET:
I_G[low-side](avg) = C_ISS × V_GS x F_S
Since the current from the gate drive comes from the
input voltage, the power dissipated in the MIC2159 due
to gate drive is:
P_GATEDRIVE = V_IN.(I_G[high-sde](avg) + I_G[low-side](avg))
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R_DS(ON) ×
October 2006
MIC2159
Q_G. Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2159.
Parameters that are important to MOSFET switch
selection are:
• Voltage rating
• On-resistance
• Total gate charge
The voltage ratings for the top and bottom MOSFET are
essentially equal to the input voltage. A safety factor of
20% should be added to the V_DS(max) of the MOSFETs
to account for voltage spikes due to circuit parasitic
elements.
The power dissipated in the switching transistor is the
sum of the conduction losses during the on-time
(P_CONDUCTION) and the switching losses that occur during
the period of time when the MOSFETs turn on and off
(P_AC).
P_SW = P_CONDUCTION + P_AC
Where:
P_CONDUCTION
=
I_SW
2
(RMS)
⋅ R_SW
P_AC = P_{AC(off )} + P_AC(on)
R_SW = on-resistance of the MOSFET switch
D = duty _ cyle = ^VOUT
V_IN
Making the assumption the turn-on and turn-off transition
times are equal; the transition times can be
approximated by:
t_T
=
C_ISS
⋅ V_GS ⋅ C_OSS
I_G
⋅ V_IN
where:
CISS and COSS are measured at VDS = 0
IG = gate-drive current (1.4A for the MIC2159)
The total high-side MOSFET switching loss is:
P_AC = (V_IN + V_D ) ⋅ I_PK ⋅ t_T ⋅ F_S
Where:
t_T = Switching transition time (~20ns)
V_D = Freewheeling diode drop (0.5v)
F_S = Switching Frequency (400kHz)
The low-side MOSFET switching losses are negligible
and can be ignored for these calculations.
Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine the
peak-to-peak inductor ripple current. Generally, higher
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