LTC1735
U
(Refer to Functional Diagram)
OPERATIO
INTVCC/EXTVCC POWER
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.
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1735 is derived from the
INTVCC pin. When the EXTVCC pin is left open, an internal
5.2V low dropout regulator supplies the INTVCC power
from VIN. If EXTVCC is raised above 4.7V, the internal
regulator is turned off and an internal switch connects
EXTVCC to INTVCC. This allows a high efficiency source,
suchastheprimaryorasecondaryoutputoftheconverter
itself, to provide the INTVCC power. Voltages up to 7V can
be applied to EXTVCC for additional gate drive capability.
PGOOD (LTC1735F Only)
A window comparator monitors the output voltage and its
open-drain output is pulled low when the divided down
outputvoltageisnotwithin±7.5%ofthereferencevoltage
of 0.8V.
U
W
U U
APPLICATIO S I FOR ATIO
ThebasicLTC1735applicationcircuitisshowninFigure 1
on the first page. External component selection is driven
by the load requirement and begins with the selection of
RSENSE. OnceRSENSE isknown, COSC andLcanbechosen.
Next, the power MOSFETs and D1 are selected. The
operating frequency and the inductor are chosen based
largelyonthedesiredamountofripplecurrent. Finally, CIN
is selected for its ability to handle the large RMS current
into the converter and COUT is chosen with low enough
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).
COSC Selection for Operating Frequency and
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 COSC. Each time the topside MOSFET turns on, the
voltage on COSC is reset to ground. During the on-time,
COSC ischargedbyafixedcurrent.Whenthevoltageonthe
capacitor reaches 1.19V, COSC is reset to ground. The
process then repeats.
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
The LTC1735 current comparator has a maximum thresh-
old of 75mV/RSENSE and an input common mode range of
SGND to 1.1(INTVCC). The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current IMAX equal to the peak value less
half the peak-to-peak ripple current, ∆IL.
The value of COSC is calculated from the desired operating
frequency assuming no external clock input on the FCB
pin:
7
1.61(10 )
Frequency
C
(pF) =
– 11
OSC
Allowing a margin for variations in the LTC1735 and
external component values yields:
A graph for selecting COSC versus frequency is shown in
Figure 2. The maximum recommended switching fre-
quency is 550kHz .
50mV
R
=
SENSE
I
MAX
1735fc
11
Linear [ Linear ]
