MIC24420 Datasheet, PDF (19/34 Pages) Micrel Semiconductor – 2.5A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
damped switch node waveform.
Figure 11. Snubber Circuit
The snubber capacitor, C_S, is charged and discharged
each switching cycle. The energy stored in C_S is
dissipated by the snubber resistor, R_S, two times per
switching period. This power is calculated in the
equation below.
P_snubber = f_S ⋅ C_S ⋅ V_IN2
Where:
f_S is the switching frequency for each phase
V_IN is the DC input voltage
Low-side MOSFET Selection
An external N-channel logic level power MOSFET must
be used for the low-side switch. The MOSFET gate to
source drive voltage of the MIC24420/MIC24421 is
regulated by an internal 5V regulator. Logic level
MOSFETs, whose operation is specified at V_GS = 4.5V
must be used. Use of MOSFETs with a lower specified
V_GS (such as 3.3V or 2.5V) are not recommended since
the low threshold can cause them to turn on when the
high-side FET is turning on. When operating the
regulator below a 6V input, connect V_DD to V_IN to prevent
the V_DD regulator from dropping out.
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
regulator’s gate drive circuit. Gate charge is a source of
power dissipation in the regulator due to the high
switching frequencies. At low output load this power
dissipation is noticeable as a reduction in efficiency. The
average current required to drive the MOSFETs is:
I_DD = Q_G ⋅ f_S
Where:
MIC24420/MIC24421
Q_G is the gate charge for both of the external MOSFETs.
This information should be obtained from the
manufacturer’s data sheet.
Since current from the gate drive is supplied by the input
voltage, power dissipated in the MIC24420/MIC24421
due to gate drive is:
P_{GATE_DRIVE} = Q_G ⋅ f_S ⋅ V_IN
Parameters that are important to MOSFET selection are:
• Voltage rating
• On resistance
• Total gate charge
The MOSFET is subjected to a V_DS equal to the input
voltage. A safety factor of 20% should be added to the
V_DS(max) of the MOSFET to account for voltage spikes
due to circuit parasitics. Generally, 30V MOSFETs are
recommended for all applications since lower V_DS rated
MOSFETs tend to have a V_GS rating that is lower than
the recommended 4.5V.
RMS Current and MOSFET Power Dissipation
Calculation
Switching loss in the low-side MOSFET can be
neglected since it is turned on and off at a V_DS of 0V.
The power dissipated in the MOSFET is mostly
conduction loss during the on-time (P_CONDUCTION).
P = I ⋅ R _CONDUCTION
2
SW_RMS
DS(ON)
Where:
R_DS(ON) is the on resistance of the MOSFET switch.
The RMS value of the MOSFET current is:
I_{SW_RMS} =
(1 − D) ⋅ (I_{OUT_MAX} ²
+
I_PP ²
12
)
Where:
D is the duty-cycle of the converter
I_PP is the inductor ripple current
D = ^VOUT
η ⋅ V_IN
Where:
η is the efficiency of the converter.
External Schottky Diode
A freewheeling diode in parallel with the low-side
MOSFET is needed to maintain continuous inductor
current flow while both MOSFETs are turned off (dead-
time). Dead-time is necessary to prevent current from
flowing unimpeded through both MOSFETs. An external
Schottky diode is used to bypass the low-side
MOSFET’s parasitic body diode. An external diode
June 2012
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M9999-062012-C