RT8024
9
DS8024-02 March 2011 www.richtek.com
Output Voltage Programming
The resistive divider allows the V
FB
pin to sense a fraction
of the output voltage as shown in Figure 4.
)
R2
R1
(1VV
REFOUT
+=
Figure 4. Setting the Output Voltage
where V
REF
is the internal reference voltage (0.6V typ.)
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as :
Efficiency = 100% − (L1+ L2+ L3+ ...)
where L1, L2, etc. are the individual losses as a percentage
of input power. Although all dissipative elements in the
circuit produce losses, two main sources usually account
for most of the losses : VIN quiescent current and I
2
R
losses. The VIN quiescent current loss dominates the
efficiency loss at very low load currents whereas the I
2
R
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve
at very low load currents can be misleading since the
actual power lost is of no consequence.
1. The VIN quiescent current is due to two components :
the DC bias current as given in the electrical characteristics
and the internal main switch and synchronous switch gate
charge currents. The gate charge current results from
switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
high to low to high again, a packet of charge ΔQ moves
from V
IN
to ground.
The resulting ΔQ/Δt is the current out of V
IN
that is typically
larger than the DC bias current. In continuous mode,
I
GATECHG
= f(Q
T
+Q
B
)
where Q
T
and Q
B
are the gate charges of the internal top
and bottom switches. Both the DC bias and gate charge
losses are proportional to V
IN
and thus their effects will
be more pronounced at higher supply voltages.
2. I
2
R losses are calculated from the resistances of the
internal switches, R
SW
and external inductor R
L
. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main switch
and the synchronous switch. Thus, the series resistance
looking into the LX pin is a function of both top and bottom
MOSFET R
DS(ON)
and the duty cycle (DC) as follows :
R
SW
= R
DS(ON)TOP
x DC + R
DS(ON)BOT
x (1−DC)
The R
DS(ON)
for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
For adjustable about voltage mode, the output voltage is
set by an external resistive divider according to the following
equation :
RT8024
GND
FB
R1
R2
V
OUT
polymer, aluminum electrolytic and ceramic capacitors are
all available in surface mount packages. Special polymer
capacitors offer very low ESR but have lower capacitance
density than other types. Tantalum capacitors have the
highest capacitance density but it is important to only
use types that have been surge tested for use in switching
power supplies. Aluminum electrolytic capacitors have
significantly higher ESR but can be used in cost-sensitive
applications provided that consideration is given to ripple
current ratings and long term reliability. Ceramic capacitors
have excellent low ESR characteristics but can have a
high voltage coefficient and audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
input, V
IN
. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V
IN
large enough to damage the
part.
