LT3959
11
3959fa
For more information www.linear.com/LT3959
Duty Cycle Consideration
Switching duty cycle is a key variable defining converter
operation. As such, its limits must be considered. Minimum
on-time is the smallest time duration that the LT3959 is
capable of turning on the internal power MOSFET. This time
is generally about 150ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LT3959 keeps the power switch off for at least
150ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
The minimum on-time and minimum off-time and the
switching frequency define the minimum and maximum
switching duty cycles a converter is able to generate:
Minimum duty cycle = minimum on-time • frequency
Maximum duty cycle = 1 – (minimum off-time • frequency)
Programming the Output Voltage
The output voltage (V
OUT
) is set by a resistor divider, as
shown in Figure 1. The positive V
OUT
and negative V
OUT
are set by the following equations:
V
OUT(POSITIVE)
= 1.6V • 1+
R1
V
OUT(NEGATIVE)
= –0.8V • 1+
R2
R1
The resistors R1 and R2 are typically chosen so that the
error caused by the current flowing into the FBX pin dur-
ing normal
operation is less than 1% (this translates to a
maximum value of R1 at about 121k).
Soft-Start
The
LT3959 contains several features to limit peak switch
currents and output voltage (V
OUT
) overshoot during
start-up or recovery from a fault condition. The primary
purpose of these features is to prevent damage to external
components or the load.
High peak switch currents during start-up may occur in
switching regulators. Since V
OUT
is far from its final value,
the feedback loop is saturated and the regulator tries to
charge the output capacitor as quickly as possible, resulting
in large peak currents. A large surge current may cause
inductor saturation or power switch failure.
LT3959 addresses this mechanism with the SS pin. As
shown in Figure 1, the SS pin reduces the internal power
MOSFET current by pulling down the V
C
pin through Q2.
In this way the SS allows the output capacitor to charge
gradually toward its final value while limiting the start-up
peak currents.
Besides start-up, soft-start can also
be triggered by
INTV
CC
undervoltage lockout and/or thermal lockout, which
causes the LT3959 to stop switching immediately. The SS
pin will be discharged by Q3. When all faults are cleared
and the SS pin has been discharged below 0.2V, a 10µA
current source I
S2
starts charging the SS pin, initiating a
soft-start operation.
The soft-start interval is set by the soft-start capacitor
selection according to the equation:
T
SS
= C
SS
•
10µA
FBX Frequency Foldback
When V
OUT
is very low during start-up or a short-circuit
fault on the output, the switching regulator must operate
at low duty cycles to maintain the power switch current
within the current limit range, since the inductor current
decay rate is very low during switch off time. The minimum
on-time limitation may prevent the switcher from attaining
a sufficiently low duty cycle at the programmed switching
frequency. So, the switch current will keep increasing
through each switch cycle, exceeding the programmed
current limit. To prevent the switch peak currents from
exceeding the programmed value, the LT3959 contains
a frequency foldback function to reduce the switching
frequency when the FBX voltage is low (see the Normal
-
ized Switching
Frequency vs FBX graph in the Typical
Performance Characteristics section).
applicaTions inForMaTion
