Datasheet

ISW
LIMIT
=
I
OUT
(1-D)
+
(D x V
IN
)
(2 x f
S
x L)
R
SEN
=
V
SENSE
- (D x V
SL
)
ISW
LIMIT
0.000 0.100 0.200 0.300 0.400 0.500
CURRENT SENSE VOLTAGE (V)
0
20
40
60
80
100
120
DUTY CYCLE (%)
F
S
=
250 kHz
V
SENSE
V
SL
F
S
= 500 kHz
LM3478
LM3478-Q1
SNVS085V –JULY 2000–REVISED FEBRUARY 2013 www.ti.com
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage can be programmed using a resistor divider between the output and the FB pin. The resistors
are selected such that the voltage at the FB pin is 1.26V. Pick R
F1
(the resistor between the output voltage and
the feedback pin) and R
F2
(the resistor between the feedback pin and ground) can be selected using the
following equation,
R
F2
= (1.26V x R
F1
) / (V
out
- 1.26V) (15)
A 100pF capacitor may be connected between the feedback and ground pins to reduce noise.
SETTING THE CURRENT LIMIT
The maximum amount of current that can be delivered to the load is set by the sense resistor, R
SEN
. Current limit
occurs when the voltage that is generated across the sense resistor equals the current sense threshold voltage,
V
SENSE
. When this threshold is reached, the switch will be turned off until the next cycle. Limits for V
SENSE
are
specified in the electrical characteristics section. V
SENSE
represents the maximum value of the internal control
signal V
CS
as shown in Figure 33. This control signal, however, is not a constant value and changes over the
course of a period as a result of the internal compensation ramp (V
SL
). Therefore the current limit threshold will
also change. The actual current limit threshold is a function of the sense voltage (V
SENSE
) and the internal
compensation ramp:
R
SEN
x ISW
LIMIT
= VCS
MAX
= V
SENSE
- (D x V
SL
) (16)
Where ISW
LIMIT
is the peak switch current limit, defined by the equation below.
Figure 33. Current Sense Voltage vs Duty Cycle
Figure 33 shows how V
CS
(and current limit threshold voltage) change with duty cycle. The curve is equivalent to
the internal compensation ramp slope (S
e
) and is bounded at low duty cycle by V
SENSE
, shown as a dotted line.
As duty cycle increases, the control voltage is reduced as V
SL
ramps up. The graph also shows the short circuit
current limit threshold of 343 mV (typical) during the 325 ns (typical) blanking time. For higher frequencies this
fixed blanking time obviously occupies more duty cycle, percentage wise. Since current limit threshold varies with
duty cycle, the following equation should be used to select R
SEN
and set the desired current limit threshold:
(17)
The numerator of the above equation is V
CS
, and ISW
LIMIT
is calculated as:
(18)
To avoid false triggering, the current limit value should have some margin above the maximum operating value,
typically 120%. Values for both V
SENSE
and V
SL
are specified in Electrical Characteristics. However, calculating
with the limits of these two specs could result in an unrealistically wide current limit or R
SEN
range. Therefore, the
following equation is recommended, using the V
SL
ratio value given in Electrical Characteristics:
16 Submit Documentation Feedback Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM3478 LM3478-Q1