LTC3808 Datasheet, PDF (18/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
V_X
V_X
V_OUT
V_OUT
TIME
(5b) Coincident Tracking
TIME
3808 F05b,c
(5c) Ratiometric Tracking
Figure 5b and 5c. Two Different Modes of Output Voltage Tracking
For coincident tracking (V_OUT = V_X during start-up),
R_TA = R_A, R_TB = R_B
V_X should always be greater than V_OUT when using the
tracking function of TRACK/SS pin.
The internal current source (1µA), which is for external
soft-start, will cause a tracking error at V_OUT. For example,
if a 59k resistor is chosen for R_TA, the R_TA current will be
about 10µA (600mV/59k). In this case, the 1µA internal
current source will cause about 10% (1µA/10µA • 100%)
tracking error, which is about 60mV (600mV • 10%)
referred to V_FB. This is acceptable for most applications. If
a better tracking accuracy is required, the value of R_TA
should be reduced.
Table 1 summarizes the different states in which the
TRACK/SS can be used.
Table 1. The States of the TRACK/SS Pin
TRACK/SS Pin
FREQUENCY
Capacitor C_SS
V_IN
Resistor Divider
External Soft-Start
Internal Soft-Start
V_OUT Tracking an External Voltage V_X
Phase-Locked Loop and Frequency Synchronization
The LTC3808 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the external P-channel
MOSFET to be locked to the rising edge of an external clock
signal applied to the SYNC/MODE pin. The phase detector
18
is an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of complemen-
tary current sources that charge or discharge the external
filter network connected to the PLLLPF pin. The relation-
ship between the voltage on the PLLLPF pin and operating
frequency, when there is a clock signal applied to SYNC/
MODE, is shown in Figure 6 and specified in the electrical
characteristics table. Note that the LTC3808 can only be
synchronized to an external clock whose frequency is
within range of the LTC3808’s internal VCO, which is
1200
1000
800
600
400
200
0
0.2
0.7
1.2
1.7
2.2
PLLLPF PIN VOLTAGE (V)
3808 F06
Figure 6. Relationship Between Oscillator Frequency
and Voltage at the PLLLPF Pin When Synchronizing to
an External Clock
3808f