LM3478MA_15 Datasheet, PDF (16/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – 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 32. 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.
120
V_SL
100
80
V_SENSE
60
F_S = 500 kHz
40
20
F_S =
250 kHz
0
0.000 0.100
0.200
0.300
0.400 0.500
CURRENT SENSE VOLTAGE (V)
Figure 32. Current Sense Voltage vs Duty Cycle
Figure 32 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:
V_SENSE - (D x V_SL)
R_SEN =
ISW_LIMIT
(17)
The numerator of the above equation is V_CS, and ISW_LIMIT is calculated as:
ISW_LIMIT =
I_{OUT +} (D x V_IN)
(1-D) (2 x f_S x L)
(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 the characteristic table. 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
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