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______________________________________________________ GENERAL OVERVIEW
voltage selection. For a low duty cycle application
MOSFET Gate Drive
The P-channel drive is derived through an
internal regulator that generates V IN -5V. This pin
(VDR) must be connected to V IN with a 0.1 μ F
decoupling capacitor. The gate drive circuit
such as the circuit shown on first page, the
Schottky diode is conducting most of the time and
its conduction losses are the largest component
of losses in the converter. Conduction losses can
be estimated from:
Pc = Vf × Iout × ? 1 ?
Vout ?
swings between V IN and V IN -5 and employs
powerful drivers for efficient switching of the P-
channel MOSFET.
Power MOSFET Selection
where:
?
?
?
Vin ?
V F is diode forward voltage at I OUT
Select the Power MOSFET for Voltage rating
RDS ( ON ) ≤ ?
L = ( Vin ? Vout ) × ?
? × ? ?
? ? × ? ?
? ?
Pcond = Iout 2 × RDS ( ON ) × ?
? Vout ?
BV DSS , On resistance R DS ( ON ), and thermal
resistance R THJA . BV DSS should be about twice
as high as V IN in order to guard against switching
transients. The recommended MOSFET voltage
rating for V IN of 5V, 12V and 24V is 12V, 30V and
40V respectively. R DS ( ON ) must be selected such
that when operating at peak current and junction
temperature, the Overcurrent threshold of the
SP6127 is not exceeded. Allowing 50% for
temperature coefficient of R DS ( ON ) and 15% for
inductor current ripple, the following expression
can be used:
? 0 . 3 V ?
?
? 1 . 5 × 1 . 15 × Iout ?
Within this constraint, selecting MOSFETs with
lower R DS ( ON ) will reduce conduction losses at
the expense of increased switching losses. As a
rule of thumb, select the highest R DS ( ON )
MOSFET that meets the above criteria. Switching
losses can be assumed to roughly equal to the
conduction losses. A simplified expression for
conduction losses is given by:
?
? Vin ?
The MOSFET’s junction temperature can be
estimated from:
T = ( 2 × Pc × Rthja ) + Tambient
Schottky Rectifier selection
Select the Schottky Diode for Voltage rating V R ,
Forward voltage V f , and thermal resistance
R THJA . The Voltage rating should be selected
using the same guidelines outlined for MOSFET
The Schottky diode’s AC losses due to its
switching capacitance are negligible.
Inductor Selection
Select the Inductor for inductance L and
saturation current I SAT . Select an inductor with
I SAT higher than the programmed overcurrent.
Calculate inductance from:
? Vout ? ? 1 ? ? 1 ?
? Vin ? ? f ? ? Irip ?
where:
V IN is converter input voltage
V OUT is converter output voltage
f is switching frequency
I RIP is inductor peak-to-peak current ripple
(nominally set to 30% of I OUT )
Keep in mind that a higher I RIP results in a
smaller inductor which has the advantages of
small size, low DC equivalent resistance DCR,
high saturation current I SAT and allows the use of
a lower output capacitance to meet a given step
load transient. A higher I RIP , however, increases
the output voltage ripple and increases the
current at which converter enters Discontinuous
Conduction Mode. The output current at which
converter enters DCM is ? of I RIP . Note that a
negative current step load that drives the
converter into DCM will result in a large output
voltage transient. Therefore the lowest current for
a step load should be larger than ? of I RIP .
Jan28-08 RevE
SP6127 TSOT-6 PFET Buck Controller
Page 7
? 2007 Exar Corporation
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