LTC3499_15 Datasheet, PDF (8/16 Pages) Linear Technology – 750mA Synchronous Step-Up DC/DC Converters with Reverse-Battery Protection

LTC3499/LTC3499B
OPERATION
The LTC3499/LTC3499B provide high efficiency, low noise
power for boost applications with output voltages up to
6V. Operation can be best understood by referring to
the Functional Block Diagram in Figure 1. The synchro-
nous boost converters are housed in either an 8-lead
(3mm × 3mm) DFN or MSOP package and operates at a
fixed 1.2MHz. With a 1.6V typical minimum V_IN voltage
these devices are well suited for applications using two
or three alkaline or nickel-metal hydride (NiMH) cells or
one Lithium-Ion (Li-Ion) cell. The LTC3499/LTC3499B
have integrated circuitry which protects the battery, IC,
and circuitry powered by the device in the event that the
input batteries are connected backwards (reverse battery
protection). The true output disconnect feature eliminates
inrush current and allows V_OUT to be zero volts during
shutdown. The current mode architecture simplifies loop
compensation with excellent load transient response.
The low R_DS(ON), low gate charge synchronous switches
eliminate the need for an external Schottky diode recti-
fier, and provide efficient high frequency pulse width
modulation (PWM). Burst Mode quiescent current to the
LTC3499 is only 20µA from V_IN, maximizing battery life.
The LTC3499B does not have Burst Mode operation and
the device continues switching at constant frequency. This
results in the absence of low frequency output ripple at
the expense of light load efficiency.
LOW NOISE FIXED FREQUENCY OPERATION
Shutdown
The LTC3499/LTC3499B are shut down by pulling SHDN
below 0.2V, and activated by pulling the pin above 1.2V.
SHDN can be driven above V_IN or V_OUT as long as it is
limited to less than the absolute maximum rating.
Soft-Start
The soft-start time is programmed with an external capaci-
tor to ground on SS. An internal current source charges
the capacitor, C_SS, with a nominal 3µA. The voltage on SS
is used to clamp the voltage on VC. The soft-start time
is given by
t_(msec) = C_SS (µF) • 200
In the event of an external shutdown or thermal shutdown
(TSD), C_SS is discharged through a nominal 5kΩ imped-
ance to GND. Once the condition is removed and SS is
discharged near ground, a soft-start will automatically
be re-initiated.
Error Amplifier
A transconductance amplifier generates an error voltage
from the difference between the positive input internally
connected to the 1.22V reference and the negative input
connected to FB. A simple compensation network is placed
from VC to ground. Internal clamps limit the minimum and
maximum error amplifier output voltage for improved large
signal transient response. A voltage divider from V_OUT to
GND programs the output voltage via FB from 2V to 6V
and is defined by the following equation:
V_OUT
=
1.22
•
₁₊


R1
R2 


Current Sensing
Lossless current sensing converts the peak current signal
into a voltage which is summed with the internal slope
compensation. This summed signal is compared to the
error amplifier output to provide a peak current control
command for the PWM. Peak switch current is limited
to 750mA minimum.
Antiringing Control
The antiringing control connects a resistor across the
inductor to damp the ringing on SW in discontinuous
conduction mode. The LC resonant ringing (L = inductor,
C_SW = capacitance on SW) is low energy, but can cause
EMI radiation if antiringing control is not present.
Zero Current Comparator
The zero current comparator monitors the inductor current
to the output and shuts off the synchronous rectifier once
this current reduces to approximately 40mA, preventing
negative inductor current.
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