lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 10

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3686A
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Programmable Undervoltage Lockout
The EN/UVLO pin can be programmed by an external re-
sistor divider between V
the resistors according to:
R4 also sets the hysteresis voltage for the programmable
UVLO:
Once V
LT3686A will enter a low quiescent current state (Iq ≈
15μA). To shutdown the LT3686A completely (Iq < 1μA),
reduce EN/UVLO pin voltage to below 0.4V.
10
Hysteresis = R4 • 2.4μA
R1= R2
R4 =R5
IN
10000
1000
⎛
⎜
⎝
drops below the programmed voltage, the
100
0.1
1.27V
10
V
0.8V
1
OUT
V
Figure 1. I
0
IN
– 1
1
– 1
2
⎞
⎟
⎠
Q
IN
vs V
EN/UVLO (V)
3
and the EN/UVLO pin. Choose
EN/UVLO
4
5
(V
6
IN
= 10V)
7
3686A F01
8
Input Voltage Range
The input voltage range for the LT3686A applications
depends on the output voltage and on the absolute maxi-
mum ratings of the V
input voltage is determined by either the LT3686A’s
minimum operating voltage of 3.6V, or by its maximum
duty cycle.
The duty cycle is the fraction of time that the internal
switch is on and is determined by the input and output
voltages:
Where V
(~0.4V) and V
(~0.67V at maximum load). This leads to a minimum input
voltage of:
DC
The boost capacitor is charged with the energy stored in the
inductor, the circuit will rely on some minimum load current
to sustain the charge across the boost capacitor.
DC =
MAX
V
IN(MIN)
can be adjusted with frequency.
V
D
IN
is the forward voltage drop of the catch diode
V
=
– V
OUT
45
40
35
30
25
20
15
10
SW
V
5
0
OUT
Figure 2. EN/UVLO Pin Current
DC
SW
0
+ V
is the voltage drop of the internal switch
MAX
+ V
+ V
D
10
IN
D
D
and BOOST pins. The minimum
– V
20
EN/UVLO (V)
D
+ V
30
SW
40
3686A F02
50
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