LT3433
U
U
U
PI FU CTIO S
SGND (Pins 1, 8, 9, 16): Low Noise Ground Reference.
VFB (Pin7):ErrorAmplifierInvertingInput.Thenoninvert-
ing input of the error amplifier is connected to an internal
1.231V reference. The VFB pin is connected to a resistor
divider from the converter output. Values for the resistor
connected from VOUT to VFB (RFB1) and the resistor con-
nectedfromVFB toground(RFB2)canbecalculatedtopro-
gram converter output voltage (VOUT) via the following
relation:
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 VBST pin
to the SW_H pin. A 1µF capacitor is generally adequate for
most applications.
Thechargeonthebootstrapcapacitorisrefreshedthrough
a diode, typically connected from the converter output
(VOUT), during the switch-off period. Minimum off-time
operationassuresthattheboostcapacitorisrefreshedeach
switch cycle. The LT3433 supports operational VBST sup-
ply voltages up to 75V (absolute maximum) as referenced
to ground.
VOUT = 1.231 • (RFB1 + RFB2)/RFB2
The VFB pin input bias current is 35nA, so use of extremely
high value feedback resistors could cause a converter
output that is slightly higher than expected. Bias current
error at the output can be estimated as:
SW_H (Pin 3): Boosted Switch Output. This is the current
returnfortheboostedswitchandcorrespondstotheemitter
of the switch transistor. The boosted switch shorts the
SW_H pin to the VIN supply when enabled. The drive cir-
cuitry for this switch is boosted above the VIN supply
through the VBST pin, allowing saturation of the switch for
maximum efficiency. The “ON” resistance of the boosted
switch is 0.8Ω.
∆VOUT(BIAS) = 35nA • RFB1
The voltage on VFB also controls the LT3433 oscillator
frequencythrougha“frequency-foldback”function.When
theVFB pinvoltageisbelow0.8V,theoscillatorrunsslower
than the 200kHz typical operating frequency. The oscilla-
torfrequencyslowswithreducedvoltageonthepin, down
to 50kHz when VFB = 0V.
The VFB pin voltage also controls switch current limit
througha“current-limitfoldback”function.AtVFB=0V,the
maximum switch current is reduced to half of the normal
value. The current limit value increases linearly until VFB
reaches 0.6V when the normal maximum switch current
level is restored. The frequency and current-limit foldback
functions add robustness to short-circuit protection and
help prevent inductor current runaway during start-up.
VIN (Pin 4): Input Power Supply. This pin supplies power
to the boosted switch and corresponds to the collector of
the switch transistor.This pin also supplies power to most
of the IC’s internal circuitry if the VBIAS pin is not driven
externally. This supply will be subject to high switching
transientcurrentssothispinrequiresahighqualitybypass
capacitor that meets whatever application-specific input
ripple current requirements exist. See Applications Infor-
mation.
SS(Pin10):SoftStart. Connectacapacitor(CSS)fromthis
pin to ground. The output voltage of the LT3433 error
amplifier corresponds to the peak current sense amplifier
output detected before resetting the switch output(s). The
soft-start circuit forces the error amplifier output to a zero
peak current for start-up. A 5µA current is forced from the
SS pin onto an external capacitor. As the SS pin voltage
ramps up, so does the LT3433 internally sensed peak cur-
rent limit. This forces the converter output current to ramp
from zero until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
BURST_EN (Pin 5): Burst Mode Enable Pin. Shorting this
pin to SGND enables Burst Mode operation. If Burst Mode
operationisnotdesired,connectingthispinto VBIASorVOUT
will disable the burst function.
VC (Pin 6): Error Amplifier Output. The voltage on the VC
pincorrespondstothemaximumswitchcurrentperoscil-
latorcycle. Theerroramplifieristypicallyconfiguredasan
integrator circuit by connecting an RC network from this
pin to ground. This circuit typically creates the dominant
polefortheconverterregulationfeedbackloop.Specificin-
tegratorcharacteristicscanbeconfiguredtooptimizetran-
sient response. See Applications Information.
3433ia
5
Linear [ Linear ]
