LTC3788 Linear Technology, LTC3788 Datasheet - Page 23

• Manufacturer Part Number: LTC3788
• Description: Dual Output Synchronous Boost Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Typically, the external clock (on PLLIN/MODE pin) input
high threshold is 1.6V, while the input low threshold is
1.2V.
Note that the LTC3788 can only be synchronized to an
external clock whose frequency is within range of the
LTC3788’s internal VCO, which is nominally 55kHz to 1MHz.
This is guaranteed to be between 75kHz and 850kHz.
Rapid phase locking can be achieved by using the FREQ pin
to set a free-running frequency near the desired synchro-
nization frequency. The VCO’s input voltage is prebiased
at a frequency corresponding to the frequency set by the
FREQ pin. Once prebiased, the PLL only needs to adjust
the frequency slightly to achieve phase lock and synchro-
nization. Although it is not required that the free-running
frequency be near external clock frequency, doing so will
prevent the operating frequency from passing through a
large range of frequencies as the PLL locks.
Table 2 summarizes the different states in which the FREQ
pin can be used.
Table 2.
Any of the Above
FREQ PIN
Resistor
INTV
0V
CC
PLLIN/MODE PIN
External Clock
DC Voltage
DC Voltage
DC Voltage
Figure 6. Relationship Between Oscillator
Frequency and Resistor Value at the FREQ Pin
50kHz to 900kHz
Phase Locked to
External Clock
1000
FREQUENCY
900
800
700
400
300
200
100
600
500
350kHz
535kHz
0
15
25
35 45 55
FREQ PIN RESISTOR (kΩ)
65 75 85 95 105 115 125
Minimum On-Time Considerations
Minimum on-time, t
that the LTC3788 is capable of turning on the bottom
MOSFET. It is determined by internal timing delays and
the gate charge required to turn on the top MOSFET. Low
duty cycle applications may approach this minimum on-
time limit.
In forced continuous mode, if the duty cycle falls below
what can be accommodated by the minimum on-time,
the controller will begin to skip cycles but the output will
continue to be regulated. More cycles will be skipped when
V
to keep the top MOSFET on continuously. The minimum
on-time for the LTC3788 is approximately 110ns.
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the effi ciency and which change would
produce the greatest improvement. Percent effi ciency
can be expressed as:
where L1, L2, etc., are the individual losses as a percent-
age of input power.
IN
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
increases. Once V
3788 F06
ON(MIN)
IN
rises above V
, is the smallest time duration
OUT
LTC3788
, the loop works
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3788f