LTC3735 Datasheet, PDF (17/32 Pages) Linear Technology – 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3735
APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (C_B,D_B) (Refer to
Functional Diagram)
External bootstrap capacitors C_B1 and C_B2 connected to the
BOOST1 and BOOST2 pins supply the gate drive voltages
for the topside MOSFETs. Capacitor C_B in the Functional
Diagram is charged though diode D_B from PV_CC when the
SW pin is low. When the topside MOSFET turns on, the
driver places the C_B voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V_IN and the BOOST pin rises to V_IN + PV_CC. The value of
the boost capacitor C_B needs to be 30 to 100 times that of
the total input capacitance of the topside MOSFET(s). The
reverse breakdown of D_B must be greater than PV_CC(MAX).
VID Output Voltage Programming
After 27µs ~ 71µs t_BOOT delay, the output voltage of the
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_VID, from V_FB to ground. The value of R_VID is controlled
by the six VID input pins. Another internal resistor, 5.33k
(R_ATTEN), completes the resistive divider. The output voltage
is thus set by the ratio of (R_VID + 5.33k) to R_VID.
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.
An internal 1.5µA current source charges up the soft-start
capacitor, C_SS. When the voltage on RUN/SS reaches 1.5V,
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_SENSE to 72mV/
R_SENSE. The output current thus ramps up slowly, elimi-
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:
( ) ^t_DELAY
=
1.5V
1.5µA
C_SS
=
1s/µF
C_SS
The time for the output current to ramp up is then:
( ) ^t_IRAMP
=
3V − 1.5V
1.5µA
C_SS
=
1s/µF
C_SS
By pulling the RUN/SS pin below 1V the LTC3735 is put
into low current shutdown (I_Q < 100µA). The RUN/SS pin
can be driven directly from logic as shown in Figure 6.
Diode D1 in Figure 6 reduces the start delay but allows
C_SS to ramp up slowly providing the soft-start function.
The RUN/SS pin has an internal 6V zener clamp (see
Functional Diagram).
PV_CC
3.3V OR 5V
RUN/SS
D1
R_SS*
C_SS
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
RUN/SS
C_SS
3735 F06
Figure 6. RUN/SS Pin Interfacing
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.
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_BOOT.
V_BOOT
=
0.6V
•
R2 •(R3+R5)
R5 •(R1+R2)
By using different R5 resistors, V_BOOT can be programmed.
3735fa
17