LT3433
APPLICATIO S I FOR ATIO
The LT3433 contains circuitry to eliminate the current limit
reduction associated with slope-compensation, or anti-
slope compensation. As the slope compensation ramp is
added to the sensed current, a similar ramp is added to the
current limit threshold reference. The end result is that
current limit is not compromised so the LT3433 can
provide full power regardless of required duty cycle.
Mode Switching
The LT3433 senses operational duty cycle by directly
monitoring V
IN
and V
OUT
. Voltage drops associated with
pass and catch diodes are estimated internally such that
mode switching occurs when the duty cycle required for
continuous buck operation is greater than 75%. If such a
condition exists, a second switch is enabled during the
switch on time, changing operation to a dual switch
bridged configuration. Because the voltage available across
the switched inductor is greater in bridged mode, duty
cycle will decrease.
The output current in bridged mode is not continuous, so
switch currents are considerably higher than while oper-
ating in buck mode. In order to maximize available output
power, continuous operation and low ripple currents are
recommended. Switch currents will increase by a factor of
1/(1 – DC) during bridged mode, so this mode of operation
is typically the gating item for converter drive capability.
I
OUT(MAX)
= I
SW(MAX)
• (1 – DC)
= [0.5A – (∆I
L
/ 2)] • (1 – DC)
where
∆I
L
is the ripple current in the inductor.
It is also important to note that I
OUT
cannot be considered
equivalent to I
LOAD
during bridged operation. Most of the
converter’s switch drive power is derived from the gener-
ated output supply, so I
OUT
must also accommodate this
current requirement. During single-switch buck opera-
tional conditions, switch drive current is negligible in
terms of output current; however, during bridged opera-
tion, these currents can become significant. These output
derived switch drive currents will increase the current
loading on V
IN
by the same 1/(1 – DC) factor as the switch
currents. As maximum switch current is referenced to that
coming from the V
IN
supply, the available maximum
U
switch current will be reduced by this required drive
current.
I
DRIVE
= DC • 2 • I
SW(MAX)
• I
SWDRIVE(MAX)
Using 50mA/A for the required drive current for each
switch yields the portion of switch current used to drive
the switches is:
I
SW(DRIVE)
= DC • 2 • I
SW(MAX)
• 0.05/(1 – DC)
Removing drive currents from the available maximum
switch current yields:
I
SW(MAX)
' = I
SW(MAX)
• [1 – DC • 2 • I
SW(MAX)
•
0.05/(1 – DC)]
where I
SW(MAX)
' is maximum switch current available to
the load during bridged operation. The maximum load
current can then be calculated as:
I
LOAD(MAX)
= I
SW(MAX)
' • (1 – DC)
which reduces to:
I
LOAD(MAX)
= [0.5A – (∆I
L
/2)] • (1 – 1.1 • DC)
Design Equations
V
IN
SW_H
LT3433
SW_L
L
V
OUT
3433 AI01
W
U U
Constants:
V
SWH
= voltage drop across boosted switch
V
SWL
= voltage drop across grounded switch
V
F
= forward drop of external Schottky diodes
f
0
= operating frequency
Duty Cycle (continuous operation):
DC
BUCK
= (V
OUT
+ 2V
F
)/(V
IN
– V
SWH
+ V
F
)
DC
BRIDGED
= (V
OUT
+ 2V
F
)/(V
OUT
+ V
IN
+ 2V
F
– V
SWH
– V
SWL
)
3433ia
9
LINER [ LINEAR TECHNOLOGY ]
