LTC3429/LTC3429B
7
3429fa
Thermal Shutdown
An internal temperature monitor will start to reduce the
peak current limit if the die temperature exceeds 125°C. If
the die temperature continues to rise and reaches 160°C,
the part will go into thermal shutdown, all switches will be
turned off and the soft-start capacitor will be reset. The
part will be enabled again when the die temperature drops
by about 15°C.
Burst Mode OPERATION (LTC3429 Only)
Portable devices frequently spend extended time in low
power or standby mode, only switching to high power
consumption when specific functions are enabled. To
improve battery life in these types of products, it is
important to maintain a high power conversion efficiency
over a wide output power range. The LTC3429 provides
automatic Burst Mode operation to increase efficiency of
the power converter at light loads. Burst Mode operation
is initiated if the output load current falls below an inter-
nally programmed threshold. This threshold has an in-
verse dependence on the duty cycle of the converter and
also the value of the external inductor (See Typical Perfor-
mance Characteristics, Output Current Burst Mode Thresh-
old vs V
IN
). Once Burst Mode operation is initiated, only
the circuitry required to monitor the output is kept alive
and the rest of the device is turned off. This is referred to
as the sleep state in which the IC consumes only 20µA
from the output capacitor. When the output voltage droops
by about 1% from its nominal value, the part wakes up and
commences normal PWM operation. The output capacitor
recharges and causes the part to re-enter the sleep state
if the output load remains less than the Burst Mode
threshold. The frequency of this intermittent PWM or
burst operation depends on the load current; that is, as the
load current drops further below the burst threshold, the
LTC3429 turns on less frequently. When the load current
increases above the burst threshold, the LTC3429
seamlessly resumes continuous PWM operation. Thus,
Burst Mode operation maximizes the efficiency at very
light loads by minimizing switching and quiescent losses.
However, the output ripple typically increases to about 2%
peak-to-peak. Burst Mode ripple can be reduced, in some
circumstances, by placing a small phase-lead capacitor
(C
PL
) between V
OUT
and FB pins (refer to the Block
Diagram). However, this may adversely affect the effi-
ciency and the quiescent current requirement at light
loads. Typical values of C
PL
range from 15pF to 220pF.
OUTPUT DISCONNECT AND INRUSH LIMITING
The LTC3429/LTC3429B are designed to allow true output
disconnect by eliminating body diode conduction of the
internal PMOS rectifier. This allows V
OUT
to go to zero
volts during shutdown, drawing zero current from the
input source. It also allows for inrush current limiting at
start-up, minimizing surge currents seen by the input
supply. Note that to obtain the advantage of output discon-
nect, there must not be an external Schottky diode con-
nected between the SWITCH pin and V
OUT
.
Board layout is extremely critical to minimize voltage
overshoot on the SWITCH pin due to stray inductance.
Keep the output filter capacitor as close as possible to the
V
OUT
pin and use very low ESR/ESL ceramic capacitors
tied to a good ground plane. For applications with V
OUT
over 4.3V, a Schottky diode is required to limit the peak
SWITCH voltage to less than 6V unless some form of
external snubbing is employed. This diode must also be
placed very close to the pins to minimize stray inductance.
See the Applications Information.
SHORT CIRCUIT PROTECTION
Unlike most boost converters, the LTC3429/LTC3429B
allow their output to be short circuited due to the output dis-
connect feature. The devices incorporate internal features
such as current limit foldback, thermal regulation and ther-
mal shutdown for protection from an excessive overload
or short circuit. In the event of a short circuit, the internal
soft-start capacitor gets partially discharged. This, in turn,
causes the maximum current limit to foldback to a smaller
value. In addition to this, a thermal regulation circuit starts
to dial back the current limit farther if the die temperature
rises above 125°C. If the die temperature still reaches
160°C, the device shuts off entirely.
V
IN
> V
OUT
OPERATION
The LTC3429/LTC3429B will maintain voltage regulation
even if the input voltage is above the output voltage. This
OPERATIO
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