Datasheet
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
Q
-V
SEN
|
V
OUT
V
IN
R
SL
LR
SN
Q
1
D
C
IN
C
OUT
R
ESR
R
R
FB1
R
FB2
V
IN
V
OUT
POWER STAGE
LM3477
DR
6
I
SEN
1
V
IN
8
GND
4
FB
3
CB
7
COMP/SD
2
SW
5
R
DR
C
BOOT
R
C
C
C1
COMPENSATION NETWORK
-
V
SEN
+
C
BYP
LOAD
LM3477
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SNVS141K –OCTOBER 2000–REVISED MARCH 2013
Figure 25. LM3477 Buck Converter Reference Schematic
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage can be programmed using a resistor divider between the output and the feedback pins, as
shown in Figure 25. The resistors are selected such that the voltage at the feedback pin is 1.27V. R
FB1
and R
FB2
can be selected using the equation:
V
OUT
= 1.27*(1+ R
FB1
/R
FB2
) (4)
CALCULATING THE DUTY CYCLE
In buck converter applications, the duty cycle of the LM3477/A may be calculated as:
(5)
Where
V
D
= forward drop of the power diode ≊ 0.5V
V
Q
= V
DS
of the MOSFET when it is conducting ≊ I
OUT
*R
DSON
V
SEN
= Voltage across the sense resistor = I
OUT
x R
SN
This is the fraction of the switching period that the switch is on. The switch is off for the remainder of the period.
This fraction is expressed as:
D' = 1 − D (6)
The LM3477/A has limits for the maximum and minimum duty cycle (see Electrical Characteristics). The
maximum duty cycle of 93% (typical) will limit how low the input voltage may drop while maintaining a regulated
output voltage (the dropout voltage). In situations where a very low dropout voltage is required, it is necessary to
include V
D
, V
Q
and V
SN
losses in the maximum duty cycle calculation. Voltage drops in the inductor will lower the
dropout voltage as well.
The LM3487 provides the FET drive voltage through the voltage developed across Cboot, which is charged when
the SW pin goes low. If Cboot cannot fully recharge, the device will automatically restart when the Cboot voltage
falls below approximately 2V. Therefore, a Cboot value of at least 0.1uF is recommended to ensure normal
operation at high duty cycles.
The minimum duty cycle of the LM3477/A corresponds to the minimum on time, or blank out time (see Electrical
Characteristics).
D
MIN
= T
MIN
* f
s
(7)
This will not limit how high the input voltage can rise, however the LM3477/A will operate in hysteretic mode once
the operating duty cycle decreases to the minimum duty cycle.
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