LTC1266-33 Datasheet, PDF (12/20 Pages) Linear Technology – Synchronous Regulator Controller for N- or P-Channel MOSFETs

LTC1266
LTC1266-3.3/LTC1266-5
UU W U
APPLICATIO S I FOR ATIO
1000
800
L = 50µH
R_SENSE = 0.02Ω
600
L = 25µH
R_SENSE = 0.02Ω
400
200
0
0
L = 50µH
R_SENSE = 0.05Ω
1
2
3
4
5
(V_IN – V_OUT) VOLTAGE (V)
LTC1266 • F04
Figure 4. Minimum Value of C_OUT
series would normally be in continuous operation. The
output remains in regulation at all times. This minimum
capacitance requirement may be relaxed if Burst Mode
operation is disabled.
N-Channel vs P-Channel MOSFETs
The LTC1266 has the capability to drive either an
N-channel or a P-channel topside switch to give the user
more flexibility. N-channel MOSFETs are superior in per-
formance to P-channel due to their lower R_DS(ON) and
lower gate capacitance and are typically less expensive;
however, they do have a slightly more complicated gate
drive requirement and a more limited input voltage range
(see following sections).
Driving P-Channel Topside MOSFETs
The P-channel topside switch circuit configuration is the
most straightforward due to the requirement of only one
supply voltage level. This is due to the negative gate
threshold of the P-channel MOSFET which allows the
MOSFET to be switched on and off by swinging the gate
between V_IN and ground. The phase invert (Pin 3) is tied
to ground to choose this operating mode. Normally, the
converter input (V_IN) is connected to the LTC1266 supply
Pins 2 and 5 and can go as high as 20V. Pin 2 supplies the
high frequency current pulses to switch the MOSFETs and
should be decoupled with a 0.1µF to 1µF ceramic capaci-
tor. Pin 5 supplies most of the quiescent power to the rest
of the chip.
Driving N-Channel Topside MOSFETs
Driving an N-channel topside MOSFET (PINV, Pin 3, tied to
PWR V_IN) is a little trickier than driving a P-channel since
the gate voltage must be positive with respect to the
source to turn it on, which means that the gate voltage
must be higher than V_IN. This requires either a second
supply at least V_GS(ON) above V_IN or a bootstrapping circuit
to boost the V_IN to the proper level. The easiest method is
using a higher supply (see Figure 14) but if one is not
available, the bootstrap method can be used at the ex-
pense of an additional diode (see Figure 1). The bootstrap
works by charging the bootstrap capacitor to V_IN during
the off-time. During the on-time, the bottom plate of the
capacitor is pulled up to V_IN so that the voltage at Pin 2 is
now twice V_IN (plus any ringing on the switch node).
Since the maximum allowable voltage at Pin 2 is 20V, the
Figure 1 bootstrap circuit limits V_IN to less than 10V. A
higher V_IN can be achieved if the bootstrap capacitor is
charged to a voltage less than V_IN, in which case
V_IN(MAX) = 20 – V_CAP.
N-channel mode, internal circuitry limits the maximum
on-time to 60µs to guarantee start-up of the bootstrap
circuit. This maximum on-time reduces the maximum
duty cycle to:
Max
Duty
Cycle
=
60µs
60µs + t_OFF
which slightly increases the minimum input voltage at
which dropout occurs. However, because of the superior
on-conductance of the N-channel, the dropout perfor-
mance of an all N-channel regulator is still better (see
Figure 5) even with the duty cycle limitation, except at light
loads.
Low-Battery Comparator
The LTC1266 has an on-chip low-battery comparator
which can be used to sense a low-battery condition when
implemented as shown in Figure 6. The resistor divider
R1, R2 sets the comparator trip point as follows:
) ^{VTRIP = 1.25}
1+
R2
R1
12