Datasheet

LT3791-1
14
37911f
applicaTions inForMaTion
The Typical Application on the front page is a basic LT3791-1
application circuit. External component selection is driven
by the load requirement, and begins with the selection of
R
SENSE
and the inductor value. Next, the power MOSFETs
are selected. Finally, C
IN
and C
OUT
are selected. This circuit
can operate up to an input voltage of 60V.
Programming The Switching Frequency
The RT frequency adjust pin allows the user to program the
switching frequency from 200kHz to 700kHz to optimize
efficiency/performance or external component size. Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
may not allow sufficiently high or low duty cycle operation.
Lower frequency operation gives better performance at the
cost of larger external component size. For an appropriate
R
T
resistor value see Table 1. An external resistor from
the RT pin to GND is required; do not leave this pin open.
Table 1. Switching Frequency vs R
T
Value
f
OSC
(kHz) R
T
(kΩ)
200 147
300 84.5
400 59.0
500 45.3
600 35.7
700 29.4
Frequency Synchronization
The LT3791-1 switching frequency can be synchronized
to an external clock using the SYNC pin. Driving SYNC
with a 50% duty cycle waveform is always a good choice,
otherwise maintain the duty cycle between 10% and 90%.
The falling edge of CLKOUT corresponds to the rising edge
of SYNC thus allowing 2-phase paralleling converters. The
rising edge of CLKOUT turns on switch M3 and the falling
edge of CLKOUT turns on switch M2.
Inductor Selection
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. The inductor
value has a direct effect on ripple current. The maximum
inductor current ripple ΔI
L
can be seen in Figure 7. This
is the maximum ripple that will prevent subharmonic
oscillation and also regulate with zero load. The ripple
should be less than this to allow proper operation over
all load currents. For a given ripple the inductance terms
in continuous mode are as follows:
L
BUCK
>
V
OUT
• V
IN(MAX)
– V
OUT
( )
• 100
f •I
OUT(MAX)
• %Ripple• V
IN(MAX)
L
BOOST
>
V
IN(MIN)
2
• V
OUT
– V
IN(MIN)
( )
• 100
f •I
OUT(MAX)
• %Ripple• V
OUT
2
where:
f is operating frequency
% ripple is allowable inductor current ripple
V
IN(MIN)
is minimum input voltage
V
IN(MAX)
is maximum input voltage
V
OUT
is output voltage
I
OUT(MAX)
is maximum output load current
For high efficiency, choose an inductor with low core
loss. Also, the inductor should have low DC resistance to
reduce the I
2
R losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a shielded inductor.
R
SENSE
Selection and Maximum Output Current
R
SENSE
is chosen based on the required output current. The
current comparator threshold sets the peak of the inductor
BG1, BG2 DUTY CYCLE (%)
50
∆
I
L
/I
SENSE(MAX)
(%)
120
160
200
90
37911 F07
80
40
100
140
180
60
20
0
6055
7065
80 85 95
75
100
BOOST ∆I
L
/
I
SENSE(MAX)
LIMIT
BUCK ∆I
L
/
I
SENSE(MAX)
LIMIT
Figure 7. Maximum Peak-to-Peak Ripple vs Duty Cycle