LTC3716 Datasheet, PDF (17/28 Pages) Linear Technology – 2-Phase, 5-Bit VID, Current Mode, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3716
APPLICATIO S I FOR ATIO
OPTIONAL EXTV_CC
CONNECTION
5V < V_SEC < 7V
+
C_IN
V_IN
TG1
LTC3716
N-CH
EXTV_CC
SW1
R6
FCB
R5
BG1
PGND
N-CH
V_IN
1N4148
V_SEC
+
1 ^RSENSE 2
T1 ₃
4
+
1µF
V_OUT
C_OUT
3716 F05a
+
C_IN
V_IN
TG1
LTC3716
N-CH
EXTV_CC
SW1
BG1
PGND
N-CH
V_IN
BAT85
+
0.22µF
BAT85
VN2222LL
BAT85
1 ^RSENSE 2
L1 ₃
4
+
V_OUT
C_OUT
3716 F05b
Figure 5a. Secondary Output Loop with EXTV_CC Connection
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
INTV_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 + V_INTVCC.
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 V_IN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC3716 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier corrects for DC drops in both the power and
ground paths. The differential amplifier output signal is
Figure 5b. Capacitive Charge Pump for EXTV_CC
divided down and compared with the internal precision
0.6V voltage reference by the error amplifier.
Output Voltage Programming
The output voltage is digitally programmed as defined in
Table 1 using the VID0 to VID4 logic input pins. The VID
logic inputs program a precision, 0.25% internal feedback
resistive divider. The LTC3716 has an output voltage
range of 0.6V to 1.75V in 25mV and 50mV steps.
Between the ATTENOUT pin and ground is a variable
resistor, R1, whose value is controlled by the five VID input
pins (VID0 to VID4). Another resistor, R2, between the
ATTENIN and the ATTENOUT pins completes the resistive
divider. The output voltage is thus set by the ratio of
(R1␣ +␣ R2) to R1.
Each VID digital input is pulled up by a 40k resistor in
series with a diode from V_BIAS. Therefore, it must be
grounded to get a digital low input, and can be either
floated or connected to V_BIAS to get a digital high input. The
series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
V_BIAS. The digital inputs accept CMOS voltage levels.
V_BIAS is the supply voltage for the VID section. It is
normally connected to INTV_CC but can be driven from
other sources. If it is driven from another source, that
source must be in the range of 2.7V to 5.5V and must be
alive prior to enabling the LTC3716.
17