LTC3802_15 Datasheet, PDF (18/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
V_OUT1
R_T4 R41 R11
L1
C_OUT1
LFF
AND
PWM
EXTREF
REF
+
–
R_T5 R51 R_B1
SAW1
CH1
DUTY CYCLE
CONTROL
1.7V +
PHASEMD _–
POWER-UP/-DOWN OUTPUTS
RUN/SS
SAW2
7µA
LFF
AND
PWM
C_SS
CH2
DUTY CYCLE
CONTROL
14µA
L2
+
C_OUT2
V_OUT2
R12 R42
EXTREF
REF
R_B2 R52
CMPIN1
V_OUT1 MUST BE HIGHER THAN V_OUT2
TRACK
R11 = R41 = R_T4 = R12 = R42
–+
FBT
R_B1 = R51, R_B2 = R52
CMPIN2
RATIOMETRIC TRACKING
R_T5 = R51
C_SS = 1µF
V_OUT1 WITH
10Ω LOAD
0.5V/DIV
V_OUT2 WITH
10Ω LOAD
COINCIDENT TRACKING
R_T5 = R52
C_SS = 1µF
V_OUT1 WITH
10Ω LOAD
0.5V/DIV
V_OUT2 WITH
10Ω LOAD
3802 F04
10ms/DIV
10ms/DIV
Figure 4. Simplified Power-Up/Power-Down Output Tracking Schematic
The QFN version of the LTC3802 provides an additional
reference pin for external ratiometric start-up. If the poten-
tial at the EXTREF pin is less than 0.6V, it overrides the
internal reference. This pin can be connected to an external
ramp to control the output slew rate. If external tracking is
not required, connect EXTREF to a potential somewhat
larger than 0.6V or short EXTREF to the RUN/SS pin. The
EXTREF pin should never be allowed to float. In the GN28
package, EXTREF is internally shorted to the RUN/SS pin.
Burst Mode Operation
The LTC3802 switcher supply has two modes of opera-
tion. Under heavy loads, it operates as a fully synchro-
nous, continuous conduction switching regulator. In this
mode of operation (continuous mode), the current in the
inductor flows in the positive direction (towards the out-
put) during the entire switching cycle, constantly supply-
ing current to the load. In this mode, the synchronous
18
switch (QB) is on whenever QT is off, so the current always
flows through a low impedance switch, minimizing volt-
age drop and power loss. This is the most efficient mode
of operation at heavy loads, where the resistive losses in
the power devices are the dominant loss term.
Continuous mode works efficiently when the load current
is greater than half of the ripple current in the inductor. In
a buck converter like the LTC3802, the average current in
the inductor (averaged over one switching cycle) is equal
to the load current. The ripple current is the difference
between the maximum and the minimum current during
a switching cycle (see Figure 5a). The ripple current
depends on inductor value, clock frequency and output
voltage, but is constant regardless of load as long as the
LTC3802 remains in continuous mode. See the Inductor
Selection section for a detailed description of ripple
current.
3802f