LTC1735 Linear Technology, LTC1735 Datasheet - Page 10

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1735
OPERATIO
Foldback Current, Short-Circuit Detection and
Short-Circuit Latchoff
The RUN/SS capacitor, C
inrush current of the switching regulator. After the con-
troller has been started and been given adequate time to
charge up the output capacitors and provide full load cur-
rent, C
output voltage falls to less than 70% of its nominal output
voltage, C
the output is in an overcurrent and/or short-circuit condi-
tion. If the condition lasts for a long enough period as
determined by the size of C
down until the RUN/SS pin voltage is recycled. This built-
in latchoff can be overridden by providing a current >5 A
at a compliance of 5V to the RUN/SS pin. This current
shortens the soft-start period but also prevents net dis-
charge of C
condition. Foldback current limiting is activated when the
APPLICATIO S I FOR ATIO
The basic LTC1735 application circuit is shown in Figure 1
on the first page. External component selection is driven
by the load requirement and begins with the selection of
R
Next, the power MOSFETs and D1 are selected. The
operating frequency and the inductor are chosen based
largely on the desired amount of ripple current. Finally, C
is selected for its ability to handle the large RMS current
into the converter and C
ESR to meet the output voltage ripple and transient speci-
fications. The circuit shown in Figure 1 can be configured
for operation up to an input voltage of 28V (limited by the
external MOSFETs).
R
R
The LTC1735 current comparator has a maximum thresh-
old of 75mV/R
SGND to 1.1(INTV
sets the peak of the inductor current, yielding a maximum
average output current I
half the peak-to-peak ripple current, I
10
SENSE
SENSE
SENSE
SS
. Once R
Selection for Output Current
is chosen based on the required output current.
is used as a short-circuit time-out circuit. If the
SS
SS
begins discharging on the assumption that
SENSE
during an overcurrent and/or short-circuit
SENSE
U
CC
U
and an input common mode range of
). The current comparator threshold
is known, C
(Refer to Functional Diagram)
MAX
OUT
SS
U
SS
, is used initially to limit the
equal to the peak value less
is chosen with low enough
, the controller will be shut
OSC
W
and L can be chosen.
L
.
U
IN
output voltage falls below 70% of its nominal level whether
or not the short-circuit latchoff circuit is enabled.
INTV
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1735 is derived from the
INTV
5.2V low dropout regulator supplies the INTV
from V
regulator is turned off and an internal switch connects
EXTV
such as the primary or a secondary output of the converter
itself, to provide the INTV
be applied to EXTV
To provide clean start-up and to protect the MOSFETs,
undervoltage lockout is used to keep both MOSFETs off
until the input voltage is above 3.5V.
Allowing a margin for variations in the LTC1735 and
external component values yields:
C
Synchronization
The choice of operating frequency and inductor value is a
trade-off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses, both gate charge loss and
transition loss. However, lower frequency operation re-
quires more inductance for a given amount of ripple
current.
The LTC1735 uses a constant frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
voltage on C
C
OSC
OSC
R
SENSE
CC
CC
OSC
is charged by a fixed current. When the voltage on the
CC
Selection for Operating Frequency and
/EXTV
IN
pin. When the EXTV
to INTV
. Each time the topside MOSFET turns on, the
. If EXTV
OSC
CC
50
I
MAX
CC
POWER
mV
is reset to ground. During the on-time,
. This allows a high efficiency source,
CC
CC
is raised above 4.7V, the internal
for additional gate drive capability.
CC
CC
power. Voltages up to 7V can
pin is left open, an internal
CC
power