LM3485_15 Datasheet, PDF (12/24 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3485
SNVS178G – JANUARY 2002 – REVISED FEBRUARY 2013
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One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance
(C_ADJ) in parallel with R_ADJ results in soft-start. C_ADJ and R_ADJ create an RC time constant forcing current limit to
activate at a lower current. The output voltage will ramp more slowly when using the soft-start functionality. There
are example start-up plots for C_ADJ equal to 1nF and 10nF in the Typical Performance Characteristics. Lower
values for C_ADJ will have little to no effect on soft-start.
EXTERNAL SENSE RESISTOR
The V_DS of a PFET will tend to vary significantly over temperature. This will result an equivalent variation in
current limit. To improve current limit accuracy an external sense resistor can be connected from V_IN to the
source of the PFET, as shown in Figure 6.
Figure 6. Current Sensing by External Resistor
PGATE
When switching, the PGATE pin swings from VIN (off) to some voltage below VIN (on). How far the PGATE will
swing depends on several factors including the capacitance, on time, and input voltage.
As shown in the Typical Performance Characteristics, PGATE voltage swing will increase with decreasing gate
capacitance. Although PGATE voltage will typically be around VIN-5V, with every small gate capacitances, this
value can increase to a typical maximum of VIN-8.3V.
Additionally, PGATE swing voltage will increase as on time increases. During long on times, such as when
operating at 100% duty cycle, the PGATE voltage will eventually fall to its maximum voltage of VIN-8.3V (typical)
regardless of the PFET gate capacitance.
The PGATE voltage will not fall below 0.4V (typical). Therefore, when the input voltage falls below approximately
9V, the PGATE swing voltage range will be reduced. At an input voltage of 7V, for instance, PGATE will swing
from 7V to a minimum of 0.4V.
DESIGN INFORMATION
Hysteretic control is a simple control scheme. However the operating frequency and other performance
characteristics highly depend on external conditions and components. If either the inductance, output
capacitance, ESR, V_IN, or Cff is changed, there will be a change in the operating frequency and output ripple.
The best approach is to determine what operating frequency is desirable in the application and then begin with
the selection of the inductor and C_OUT ESR.
INDUCTOR SELECTION (L1)
The important parameters for the inductor are the inductance and the current rating. The LM3485 operates over
a wide frequency range and can use a wide range of inductance values. A good rule of thumb is to use the
equations used for Simple Switchers®. The equation for inductor ripple (Δi) as a function of output current (I_OUT)
is:
for I_out < 2.0Amps
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