LTC3735 LINER [Linear Technology], LTC3735 Datasheet - Page 17

• Manufacturer Part Number: LTC3735
• Description: 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (C
Functional Diagram)
External bootstrap capacitors C
BOOST1 and BOOST2 pins supply the gate drive voltages
for the topside MOSFETs. Capacitor C
Diagram is charged though diode D
SW pin is low. When the topside MOSFET turns on, the
driver places the C
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
the boost capacitor C
the total input capacitance of the topside MOSFET(s). The
reverse breakdown of D
VID Output Voltage Programming
After 27µs ~ 71µs t
regulator is digitally programmed as defined in Table 2
using the VID0 to VID5 logic input pins. The VID logic
inputs program a precision, 0.25% internal feedback re-
sistive divider. The LTC3735 has an output voltage range
of 0.700V to 1.708V in 16mV steps.
Refering to the Functional Diagram, there is a resistor,
R
by the six VID input pins. Another internal resistor, 5.33k
(R
is thus set by the ratio of (R
Each VID digital pin is a high impedance input. There-
fore they must be actively pulled high or pulled low. The
logic low threshold of the VID pins is 0.3V; the logic high
threshold is 0.7V.
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) run/shutdown,
2) soft-start and 3) an optional short-circuit latchoff timer.
Soft-start reduces the input power sources’ surge currents
by gradually increasing the controller’s current limit. The
latchoff timer prevents very short, extreme load transients
from tripping the overcurrent latch. A small pull-up cur-
rent (>5µA) supplied to the RUN/SS pin will prevent the
overcurrent latch from operating. The following paragraph
describes how the functions operate.
IN
VID
ATTEN
and the BOOST pin rises to V
, from V
), completes the resistive divider. The output voltage
FB
to ground. The value of R
B
BOOT
voltage across the gate-source of the
B
needs to be 30 to 100 times that of
B
must be greater than PV
delay, the output voltage of the
VID
B1
+ 5.33k) to R
and C
IN
B
B
+ PV
,D
from PV
B
B2
B
in the Functional
) (Refer to
connected to the
CC
VID
. The value of
is controlled
CC
VID
when the
.
CC(MAX).
An internal 1.5µA current source charges up the soft-start
capacitor, C
the controller is permitted to start operating. As the volt-
age on RUN/SS increases from 1.5V to 3.0V, the internal
current limit is increased from 25mV/R
R
nating the starting surge current required from the input
power supply. If RUN/SS has been pulled all the way to
ground there is a delay before starting of approximately:
The time for the output current to ramp up is then:
By pulling the RUN/SS pin below 1V the LTC3735 is put
into low current shutdown (I
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
C
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
Start-Up Sequence (Refer to the Functional Diagram)
After soft-start, the output voltage of the regulator settles
at a voltage level equal to V
By using different R5 resistors, V
SS
SENSE
t
t
V
DELAY
IRAMP
to ramp up slowly providing the soft-start function.
BOOT
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
. The output current thus ramps up slowly, elimi-
= 0.6V •
=
=
SS
1.5µA
3V − 1.5V
1.5V
. When the voltage on RUN/SS reaches 1.5V,
Figure 6. RUN/SS Pin Interfacing
1.5µA
D1
PV
R2 • R3 +R5
R5 • R1+R2
CC
C
R
SS
SS
*
C
(
RUN/SS
(
= 1s/µF
SS
(
= 1s/µF
BOOT
C
Q
SS
(
< 100µA). The RUN/SS pin
BOOT
)
)
.
)
C
SS
)
can be programmed.
C
SS
LTC3735
SENSE
RUN/SS
to 72mV/
3735 F06
C
17
SS
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