MIC2155_0911 Datasheet, PDF (25/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
The average current required to drive the MOSFETs is:
I_DD = Qg × fs
Where:
Qg is the total gate charge for all high and low side
MOSFETs. This information should be obtained from
the manufacturer’s data sheet with a 5V V_GS.
Since the current from the gate drive comes from the
input voltage, the power dissipated in the MIC2155 due
to gate drive is:
P_{GATE _ DRIVE} = Qg × f_S × V_IN
A convenient figure of merit for switching MOSFETs is
the on-resistance times the total gate charge (R_DSON ×
Qg). Lower numbers translate into higher efficiency. Low
gate charge, logic level MOSFETs are a good choice for
use with the MIC2155. The internal LDO that supplies
V_DD is rated for 75mA. Exceeding this value could
damage the regulator or cause excessive power
dissipation in the IC. Refer to the “Supply Voltages and
Internal Regulator” section of this specification for
additional information.
Parameters that are important to MOSFET switch
selection are:
• Voltage rating
• On resistance
• Total Gate Charge
The V_DS voltage rating of the MOSFETs is 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.
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_SWITCH(rms)2 × R_SWITCH
P_AC = P_{AC(off )} + P_AC(on)
R_SWITCH is the on resistance of the MOSFET switch.
MIC2155/2156
Making the assumption the turn-on and turn-off transition
times are equal, the total AC switching loss is:
P_AC = (V_IN +V_D ) × I_{SW _ PEAK} × T_t × f_S
Where:
T_t is the switching transition time (typically 15ns to
30ns)
f_S it the switching frequency of each phase
RMS Current and MOSFET Power Dissipation
Calculation
Under normal operation, the high side MOSFET’s RMS
current is greatest when V_IN is low (maximum duty
cycle). The low side MOSFET’s RMS current is greatest
when V_IN is high (minimum duty cycle). However, the
MOSFET sees maximum stress during short circuit
conditions, where the output current is equal to the
maximum overcurrent level. The calculations below are
for normal operation. To calculate the stress under short
circuit conditions, substitute the maximum overcurrent
level for I_OUT(max).
The RMS value of the high side switch current is:
I_{SW _ RMS(HIGH _ SIDE)} = D
(I_OUT(max)2
+
I_PP2
12
)
( ) I_{SW _ RMS(LOW _ SIDE)} = 1− D
(I_OUT(max)2
+
I_PP2
12
)
Where:
D is the duty cycle of the converter
I_PP is the individual inductor ripple current
D = ^VOUT
η × V_IN
and η is the efficiency of the converter.
Converter efficiency also depends on other component
parameters that have not yet been selected. For design
purposes, an efficiency estimate of 85% – 90% can be
used. The efficiency can be more accurately calculated
once the design is complete. If the assumed efficiency is
grossly inaccurate, a second iteration through the design
procedure should be made.
For the high-side switch, the maximum DC power
dissipation is:
( ) P_SWITCH1(DC) = R_DSON1 × I_SW1(rms) ²
November 2009
25
M9999-111209-B