LTC3788_15 Datasheet, PDF (22/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
Applications Information
Table 2 summarizes the different states in which the FREQ
pin can be used.
Table 2.
FREQ PIN
PLLIN/MODE PIN
FREQUENCY
0V
DC Voltage
350kHz
INTV_CC
Resistor
DC Voltage
DC Voltage
535kHz
50kHz to 900kHz
Any of the Above
External Clock
Phase Locked to
External Clock
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time duration
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_IN increases. Once V_IN rises above V_OUT, the loop works
to keep the top MOSFET on continuously. The minimum
on-time for the LTC3788 is approximately 110ns.
Efficiency Considerations
The percent efficiency 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 efficiency and which change would
produce the greatest improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3788 circuits: 1) IC V_IN current, 2) INTV_CC
regulator current, 3) I²R losses, 4) Bottom MOSFET
transition losses.
22
1. The V_IN current is the DC supply current given in the
Electrical Characteristics table, which excludes MOSFET
driver and control currents. V_IN current typically results
in a small (<0.1%) loss.
2. INTV_CC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOS-
FETs. Each time a MOSFET gate is switched from low to
high to low again, a packet of charge, dQ, moves from
INTV_CC to ground. The resulting dQ/dt is a current out
of INTV_CC that is typically much larger than the control
circuit current. In continuous mode, I_GATECHG = f(Q_T +
Q_B), where Q_T and Q_B are the gate charges of the topside
and bottom side MOSFETs.
3. DC I²R losses. These arise from the resistances of the
MOSFETs, sensing resistor, inductor and PC board traces
and cause the efficiency to drop at high output currents.
4. Transition losses apply only to the bottom MOSFET(s),
and become significant only when operating at low
input voltages. Transition losses can be estimated from:
Transition Loss = (1.7) ^VOUT3
V_IN
I_O(MAX) • C_RSS f
Other hidden losses, such as copper trace and internal
battery resistances, can account for an additional 5% to
10% efficiency degradation in portable systems. It is very
important to include these system-level losses during the
design phase.
1000
900
800
700
600
500
400
300
200
100
0
15 25 35 45 55 65 75 85 95 105 115 125
FREQ PIN RESISTOR (kΩ)
3788 F06
Figure 6. Relationship Between Oscillator
Frequency and Resistor Value at the FREQ Pin
3788fc