MIC2169 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC

MIC2169
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 applica-
tions when V_IN > 5V; logic-level MOSFETs, whose operation
is specified at V_GS = 4.5V must be used.
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in junc-
tion 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 MIC2169 gate-drive circuit. At
500kHz switching frequency and above, the gate charge can
be a significant source of power dissipation in the MIC2169.
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 × 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 V_GS = 5V.
The low-side MOSFET is turned on and off at V_DS = 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 CISS at V_DS =
0 instead of gate charge.
For the low-side MOSFET:
I_{G[low-side](avg)} = C_ISS × V_GS × f_S
Since the current from the gate drive comes from the input
voltage, the power dissipated in the MIC2169 due to gate
drive is:
( ) ^P_GATEDRIVE ^{= V}_IN ^I_{G[high-side](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) × Q_G. Lower
numbers translate into higher efficiency. Low gate-charge
logic-level MOSFETs are a good choice for use with the
MIC2169.
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 parasitics.
Micrel
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(rms)2 × R_SW
P_AC = P_AC(off) + P_AC(on)
R_SW = on-resistance of the MOSFET switch
D
=
duty
cycle


V_O
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:
C_ISS and C_OSS are measured at V_DS = 0
I_G = gate-drive current (1A for the MIC2169)
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 (typically 20ns to 50ns)
V_D = freewheeling diode drop, typically 0.5V
f_S it the switching frequency, nominally 500kHz
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 induc-
tor ripple current. Generally, higher inductance values are
used with higher input voltages. Larger peak-to-peak ripple
currents will increase the power dissipation in the inductor
and MOSFETs. Larger output ripple currents will also require
more output capacitance to smooth out the larger ripple
current. Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more expensive
inductor. A good compromise between size, loss and cost is
to set the inductor ripple current to be equal to 20% of the
maximum output current. The inductance value is calculated
by the equation below.
L = ^{VOUT × (VIN(max) − VOUT )}
V_IN(max) × f_S × 0.2 × I_OUT (max)
where:
f_S = switching frequency, 500kHz
0.2 = ratio of AC ripple current to DC output current
V_IN(max) = maximum input voltage
M9999-111803
8
November 2003