LTC1736IG Linear Technology, LTC1736IG Datasheet - Page 10

• Manufacturer Part Number: LTC1736IG
• Description: IC REG SW SYNC STEPDWN HE 24SSOP
• Manufacturer: Linear Technology

Specifications of LTC1736IG

• Applications: Converter, Intel Pentium® II, III
• Voltage - Input: 3.5 ~ 36 V
• Number Of Outputs: 1
• Voltage - Output: 0.93 ~ 2 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
LTC1736
OPERATIO
INTV
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1736 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 notebook main 5V system supply or a second-
ary output of the converter itself, to provide the INTV
power. Voltages up to 7V can be applied to EXTV
additional gate drive capability.
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.
The basic LTC1736 application circuit is shown in
Figure 1 on the first page of this data sheet. External
component selection is driven by the load requirement
and begins with the selection of R
known, C
FETs and D1 are selected. The operating frequency and the
inductor are chosen based largely on the desired amount
of ripple current. Finally, C
handle the large RMS current into the converter and C
is chosen with low enough ESR to meet the output voltage
ripple and transient specifications. 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 LTC1736 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
Allowing a margin for variations in the LTC1736 and
external component values yields:
10
SENSE
SENSE
CC
CC
CC
/EXTV
IN
pin. When the EXTV
Selection For Output Current
is chosen based on the required output current.
to INTV
. If EXTV
OSC
and L can be chosen. Next, the power MOS-
CC
SENSE
CC
Power
U
CC
. This allows a high efficiency source,
CC
U
and an input common mode range of
). The current comparator threshold
is raised above 4.7V, the internal
(Refer to Functional Diagram)
MAX
U
IN
CC
equal to the peak value less
is selected for its ability to
pin is left open, an internal
SENSE
W
L
. Once R
.
U
CC
SENSE
power
CC
OUT
for
CC
is
VID Control
Bits VID0 to VID4 are logic inputs setting the output volt-
age using an internal 5-bit DAC as a feedback resistive
voltage divider. The output voltage can be set in 50mV or
25mV increments from 0.925V to 2.0V according to
Table 1. Pins VID0 to VID4 are internally pulled up to
VIDV
PGOOD
A window comparator monitors the output voltage and its
open-drain output is pulled low when the divided down
output voltage is not within 7.5% of the reference voltage
of 0.8V.
C
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 LTC1736 uses a constant-frequency architecture with
the frequency determined by an external oscillator capaci-
tor C
voltage on C
C
capacitor reaches 1.19V, C
process then repeats.
The value of C
frequency assuming no external clock input on the FCB
pin:
OSC
OSC
R
SENSE
CC
OSC
is charged by a fixed current. When the voltage on the
Selection for Operating Frequency
.
. Each time the topside MOSFET turns on, the
50
I
OSC
MAX
OSC
mV
is reset to ground. During the on-time
is calculated from the desired operating
OSC
is reset to ground. The