LTC3499 Datasheet, PDF (9/16 Pages) Linear Technology – 750mA Synchronous Step-Up DC/DC Converters with Reverse-Battery Protection

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OPERATIO
Reverse-Battery Protection
Plugging the battery in backwards poses a severe problem
to most power converters. At a minimum the battery will
be quickly discharged. Almost all ICs have an inherent
diode from V_IN (cathode) to ground (anode) which con-
ducts appreciable current when V_IN drops more than 0.7V
below ground. Under this condition the integrated circuit
will most likely be damaged due to the excessive current
draw. There exists the possibility for the battery and
circuitry powered by the device to also be damaged. The
LTC3499/LTC3499B have integrated circuitry which al-
lows negligible current flow under a reverse-battery con-
dition, protecting the battery, device and circuitry attached
to the output. A graph of the reverse-battery current drawn
is shown in the Typical Performance Characteristics.
Discrete methods of reverse battery protection put addi-
tional dissipative elements in the high current path reduc-
ing efficiency while increasing component count to
implement protection. The LTC3499/LTC3499B do not
suffer from either of these drawbacks.
Burst Mode Operation (LTC3499 only)
Portable devices frequently spend extended time in low
power or stand-by mode, only drawing high power when
specific functions are enabled. In order to improve battery
life in these types of products, high power converter
efficiency needs to be maintained over a wide output
power range. In addition to its high efficiency at moderate
and heavy loads, the LTC3499 includes automatic Burst
Mode operation that improves efficiency of the power
converter at light loads. Burst Mode operation is initiated
if the output load current falls below an internally pro-
grammed threshold (see Typical Performance graph,
Output Load Burst Mode Threshold vs V_IN). Once initiated
the Burst Mode operation circuitry shuts down most of the
circuitry in the LTC3499, only keeping alive the circuitry
required to monitor the output voltage.
This state is referred to as sleep. In sleep, the LTC3499
only draws 20µA from the input supply, greatly enhancing
efficiency. When the output has drooped approximately
1% from its nominal regulation point, the LTC3499 wakes
up and commences normal PWM operation. The output
capacitor will recharge causing the LTC3499 to re-enter
LTC3499/LTC3499B
sleep if the output load remains less than the sleep
threshold. The frequency of this intermittent PWM (or
burst) operation is proportional to load current. Therefore,
as the load current drops further below the burst thresh-
old, the LTC3499 operates in PWM mode less frequently.
When the load current increases above the burst thresh-
old, the LC3499 will resume continuous PWM operation
seamlessly.
Referring to the Functional Block Diagram, an optional
capacitor, C_FF, between V_OUT and FB in some circum-
stances can reduce peak-to-peak V_OUT ripple and input
quiescent current during Burst Mode operation. Typical
values for C_FF range from 10pF to 220pF.
Output Disconnect and Inrush Current Limiting
The LTC3499/LTC3499B are designed to allow true output
disconnect by eliminating body diode conduction of the
internal P-channel MOSFET transistor. This allows V_OUT to
go to zero volts during shutdown without drawing any
current from the input source. It also provides for inrush
current limiting at turn-on, minimizing surge current seen
by the input supply.
V_IN > V_OUT Operation
The LTC3499/LTC3499B will maintain voltage regulation
when the input voltage is above the output voltage. This
is achieved by terminating the switching on the synchro-
nous P-channel MOSFET and applying V_IN statically on
the gate. This will ensure the volts • seconds of the
inductor will reverse during the time current is flowing to
the output. Since this mode will dissipate more power in
the IC, the maximum output current is limited in order to
maintain an acceptable junction temperature:
( ) ^{IOUT(MAX) â‰}
θ_JA
•
125 – T_A
(V_IN + 1.5) – V_OUT
where T_A = ambient temperature and θ_JA is the package
thermal resistance (45°C/W for the DD8 and 160°C/W for
the MS8).
For example at V_IN = 4.5V, V_OUT = 3.3V and T_A = 85°C in
the DD8 package, the maximum output current is 330mA.
3499f
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