LT3500IDD#PBF Linear Technology, LT3500IDD#PBF Datasheet - Page 18

• Manufacturer Part Number: LT3500IDD#PBF
• Description: IC REG STP-DWN 2A 12-DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3500IDD#PBF

• Topology: Step-Down (Buck) (1), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 2
• Frequency - Switching: 500kHz ~ 2.4MHz
• Voltage/current - Output 1: 0.8 ~ 38.9 V, 2A
• Voltage/current - Output 2: Adjustable, 13mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 3 V ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 12-DFN
• Current - Output: 2A
• Voltage - Output: 0.8 ~ 38.9 V
• Voltage - Input: 3 ~ 36 V
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3500
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 2.5V, as
in Figure 5d. The diode is used to prevent damage to the
LT3500 in case V
circuit eliminates a capacitor, but effi ciency may be lower
and dissipation in the LT3500 may be higher. Also, if V
absent, the LT3500 will still attempt to regulate the output,
but will do so with very low effi ciency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
The minimum input voltage of an LT3500 application is
limited by the minimum operating voltage (<2.8V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly, or
the LT3500 is turned on with its SS pin when the output
is already in regulation, then the boost capacitor may not
be fully charged. Because the boost capacitor is charged
with the energy stored in the inductor, the circuit will rely
on some minimum load current to get the boost circuit
running properly. This minimum load will depend on
input and output voltages and on the arrangement of the
boost circuit.
The Typical Performance Characteristics section shows
plots of the minimum load current to start and to run as a
function of input voltage for 3.3V and 5V outputs. In many
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
Use a Schottky diode for the lowest start-up voltage.
APPLICATIONS INFORMATION
18
X
is held low while V
R
C
C
C
IN
C
is ramping very slowly.
F
LT3500
V
C
IN
CURRENT MODE
POWER STAGE
g
is present. The
m
= 3mho
Figure 6. Model for Loop Response
4M
g
m
ERROR AMP
= 250μmhos
–
+
X
is
0.8V
SW
FB
Frequency Compensation
The LT3500 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3500 does not require the ESR of the output capacitor
for stability so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
ground determine loop gain. In addition, there is a lower
value capacitor in parallel. This capacitor is not part of
the loop compensation but is used to fi lter noise at the
switching frequency.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature.
The LT1375 data sheet contains a more thorough discus-
sion of loop compensation and describes how to test the
stability using a transient load.
Figure 6 shows an equivalent circuit for the LT3500 control
loop. The error amp is a transconductance amplifi er with
fi nite output impedance. The power section, consisting
of the modulator, power switch, and inductor, is modeled
as a transconductance amplifi er generating an output
C
pin. Generally a capacitor and a resistor in series to
R1
R2
TANTALUM
POLYMER
OR
C
PL
ESR
C1
V
OUT1
CERAMIC
3500 F06
C1
3500fc