LTC3499/LTC3499B
9
3499fc
OPERATION
Reverse-Battery Protection
Connecting the battery 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 conducts ap-
preciable 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 allows negligible
current flow under a reverse-battery condition, 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 ad-
ditional dissipative elements in the high current path
reducing 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 ef-
ficiency 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 programmed
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, keeping alive only 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
sleep if the output load current 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 threshold, the LTC3499 operates in PWM mode
less frequently. When the load current increases above
the burst threshold, 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 circumstances
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 switch. 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 synchronous
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:
I
OUT(MAX)
≅
125 – T
A
θ
JA
• 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.
