LTC3810-5
30
38105fd
If the external frequency (f
MODE
) is greater than the
oscillator frequency f
O
, current is sourced continuously,
pulling up the PLL/LPF pin. When the external frequency
is less than f
O
, current is sunk continuously, pulling down
the PLL/LPF pin. If the external and internal frequencies
are the same but exhibit a phase difference, the current
sources turn on for an amount of time corresponding to
the phase difference. Thus the voltage on the PLL/LPF
pin is adjusted until the phase and frequency of the external
and internal oscillators are identical. At this stable operating
point the phase comparator output is open and the filter
capacitor C
LP
holds the voltage. The LTC3810-5 MODE/
SYNC pin must be driven from a low impedance source
such as a logic gate located close to the pin.
The loop filter components (C
LP
, R
LP
) smooth out the
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components C
LP
and R
LP
determine how fast the loop
acquires lock. Typically R
LP
= 10kΩ and C
LP
is 0.01µF
to 0.1µF.
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 most improvement. Although all dissipative
elements in the circuit produce losses, four main sources
account for most of the losses in LTC3810-5 circuits:
1. DC I
2
R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the
two MOSFETs have approximately the same R
DS(ON)
,
then the resistance of one MOSFET can simply be
summed with the resistances of L and the board traces
to obtain the DC I
2
R loss. For example, if R
DS(ON)
=
0.01Ω and R
L
= 0.005Ω, the loss will range from 15mW
to 1.5W as the output current varies from 1A to 10A.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is signifi-
cant at input voltages above 20V and can be estimated
from the second term of the P
MAIN
equation found in
the Power MOSFET Selection section. When transition
losses are significant, efficiency can be improved by
lowering the frequency and/or using a top MOSFET(s)
with lower C
RSS
at the expense of higher R
DS(ON)
.
3. INTV
CC
/DRV
CC
current. This is the sum of the MOSFET
driver and control currents. Control current is typically
about 3mA and driver current can be calculated by:
I
GATE
= f(Q
G(TOP)
+ Q
G(BOT)
), where Q
G(TOP)
and Q
G(BOT)
are the gate charges of the top and bottom MOSFETs.
This loss is proportional to the supply voltage that
INTV
CC
/DRV
CC
is derived from, i.e., V
IN
for the external
NMOS linear regulator, V
OUT
for the internal EXTV
CC
regulator, or V
EXT
when an external supply is connected
to INTV
CC
/DRV
CC
.
4. C
IN
loss. The input capacitor has the difficult job of fil-
tering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in fuses or
batteries.
Other losses, including C
OUT
ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss. When
making adjustments to improve efficiency, the input cur-
rent is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by look-
ing at the load transient response. Switching regulators
take several cycles to respond to a step in load current.
applicaTions inForMaTion