LTC3838-2_15 Datasheet, PDF (26/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
To maintain a good signal-to-noise ratio for the current
sense signal, start with a ∆V_SENSE of 10mV. For a DCR
sensing application, the actual ripple voltage will be de-
termined by:
∆V_SENSE
=
V_IN – V_OUT
R1• C1
•
V_OUT
V_IN • f
Power MOSFET Selection
Two external N-channel power MOSFETs must be selected
for each channel of the LTC3838-2 controller: one for the
top (main) switch and one for the bottom (synchronous)
switch. The gate drive levels are set by the DRV_CC voltage.
This voltage is typically 5.3V. Pay close attention to the
BV_DSS specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R_DS(ON), Miller capacitance, C_MILLER, input
voltage and maximum output current. Miller capacitance,
C_MILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data sheet.
C_MILLER is equal to the increase in gate charge along the
horizontal axis while the curve is approximately flat (or
the parameter Q_GD if specified on a manufacturer’s data
sheet), divided by the specified V_DS test voltage:
C_MILLER
≅
Q_GD
V_{DS( TEST )}
When the IC is operating in continuous mode, the duty
cycles for the top and bottom MOSFETs are given by:
D_TOP
=
V_OUT
V_IN
D_BOT
= 1–
V_OUT
V_IN
The MOSFET power dissipations at maximum output
current are given by:
( ) ^P_TOP ^{= D}_TOP ^{• I}_OUT(MAX)2 ^{• R}_DS(ON)(MAX) ^{1+ δ + V}_IN2
• ^{ IOUT(}₂^{MAX )}



• C_MILLER



R_TG(UP)
V_DRVCC – V_MILLER
+
R_TG(DOWN)
V_MILLER

^•

f
 P_BOT = D_BOT • I_OUT(MAX)2 • R_DS(ON)(MAX) • (1 + δ )
where δ is the temperature dependency of R_DS(ON), R_TG(UP)
is the TG pull-up resistance, and R_TG(DOWN) is the TG pull-
down resistance. V_MILLER is the Miller effect V_GS voltage
and is taken graphically from the MOSFET ’s data sheet.
Both MOSFETs have I²R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V_IN < 20V,
the high current efficiency generally improves with larger
MOSFETs, while for V_IN > 20V, the transition losses rapidly
increase to the point that the use of a higher R_DS(ON) device
with lower C_MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs temperature curve in the
power MOSFET data sheet. For low voltage MOSFETs,
0.5% per degree (°C) can be used to estimate δ as an
approximation of percentage change of R_DS(ON):
δ = 0.005/°C • (T_J – T_A)
where T_J is estimated junction temperature of the MOSFET
and T_A is ambient temperature.
26
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38382f