LTC3499 Datasheet, PDF (8/16 Pages) Linear Technology – 750mA Synchronous Step-Up DC/DC Converters with Reverse-Battery Protection

LTC3499/LTC3499B
U
OPERATIO
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 synchronous
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+) cell. The LTC3499/LTC3499B have inte-
grated circuitry which protects the battery, IC, and cir-
cuitry powered by the device in the event that the input
batteries are connected backwards (reverse battery pro-
tection). 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 rectifier,
and provide efficient high frequency pulse width modula-
tion (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 a commanded shutdown or thermal
shutdown (TSD), C_SS is discharged through a nominal
5kΩ impedance to GND. Once the condition is removed
and SS is discharged near ground, a soft-start will auto-
matically 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 di-
vider 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 cur-
rent to the output and shuts off the synchronous rectifier
once this current reduces to approximately 40mA, pre-
venting negative inductor current.
3499f
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