Datasheet

5
LT3433
3433f
UU
U
PI FU CTIO S
the switch transistor.This pin also supplies power to most
of the IC’s internal circuitry if the V
BIAS
pin is not driven
externally. This supply will be subject to high switching
transient currents so this pin requires a high quality bypass
capacitor that meets whatever application-specific input
ripple current requirements exist.
BURST_EN (Pin 5): Burst Mode Enable/Disable. When
this pin is below 0.3V, Burst Mode operation is enabled.
Pin input bias current < 1µA when Burst Mode operation
is enabled. If Burst Mode operation is not desired, pulling
this pin above 2V will disable the burst function. When
Burst Mode operation is disabled, typical pin input current
= 35µA. BURST_EN should not be pulled above 20V. This
pin is typically shorted to SGND for Burst Mode function,
or connected to either V
BIAS
or V
OUT
to disable Burst Mode
operation.
V
C
(Pin 6): Error Amplifier Output. The voltage on the V
C
pin corresponds to the maximum switch current per oscil-
lator cycle. The error amplifier is typically configured as an
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
pole for the converter regulation feedback loop. Specific in-
tegrator characteristics can be configured to optimize tran-
sient response. See Applications Information.
SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.
V
BST
(Pin 2): Boosted Switch Supply. This “boosted” sup-
ply rail is referenced to the SW_H pin. Supply voltage is
maintained by a bootstrap capacitor tied from the V
BST
pin
to the SW_H pin. A 1µF capacitor is generally adequate for
most applications.
The charge on the bootstrap capacitor is refreshed through
a diode, typically connected from the converter output
(V
OUT
), during the switch-off period. Minimum off-time
operation assures that the boost capacitor is refreshed each
switch cycle. The LT3433 supports operational V
BST
sup-
ply voltages up to 75V (absolute maximum) as referenced
to ground.
SW_H (Pin 3): Boosted Switch Output. This is the current
return for the boosted switch and corresponds to the emitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the V
IN
supply when enabled. The drive cir-
cuitry for this switch is boosted above the V
IN
supply
through the V
BST
pin, allowing saturation of the switch for
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8.
V
IN
(Pin 4): Input Power Supply. This pin supplies power
to the boosted switch and corresponds to the collector of
TYPICAL PERFOR A CE CHARACTERISTICS
UW
V
BST
Supply Switch Drive Current
vs Temperature (I
SW
= 500mA)
Switch Resistance
vs Temperature (I
SW
= 500mA)
TEMPERATURE (°C)
–50
0.8
0.9
1.1
25 75
3433 G12
0.7
0.6
–25 0
50 100 125
0.5
0.4
1.0
SWITCH ON RESISTANCE ()
R
SWH
R
SWL
TEMPERATURE (°C)
–50 –25
I
BST
/I
SW
(mA/A)
31
34
3433 G13
28
25
0
50
10025
75
125
40
37
TEMPERATURE (°C)
–50
I
VOUT
/I
SW
(mA/A)
31
34
3433 G14
28
25
0
50
100
–25
25
75
125
40
37
V
OUT
Supply Switch Drive Current
vs Temperature (I
SW
= 500mA)