LTC3872-1_15 Datasheet, PDF (10/20 Pages) Linear Technology – No RSENSE Current Mode Boost DC/DC Controller

LTC3872-1
Applications Information
to 30V or less, and the switch node can ring during the
turn-off of the MOSFET due to layout parasitics. Check
the switching waveforms of the MOSFET directly across
the drain and source terminals using the actual PC board
layout (not just on a lab breadboard!) for excessive ringing.
During the switch on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to about
285mV, 105mV and 185mV at low duty cycle with IPRG
tied to V_IN, GND, or left floating respectively. The peak
inductor current is therefore limited to (285mV, 105mV and
185mV)/R_DS(ON) depending on the status of the IPRG pin.
The relationship between the maximum load current, duty
cycle and the R_DS(ON) of the power MOSFET is:
R_DS(ON)
≤
V_SENSE(MAX)
•
₁₊

1– D_MAX
χ
2
•I_O(MAX)
•
ρ_T
V_SENSE(MAX) is the maximum voltage drop across the
power MOSFET. V_SENSE(MAX) is typically 285mV, 185mV and
105mV. It is reduced with increasing duty cycle as shown
in Figure 3. The r_T term accounts for the temperature co-
efficient of the R_DS(ON) of the MOSFET, which is typically
0.4%/°C. Figure 4 illustrates the variation of normalized
R_DS(ON) over temperature for a typical power MOSFET.
Another method of choosing which power MOSFET to
use is to check what the maximum output current is for a
given R_DS(ON), since MOSFET on-resistances are available
in discrete values.
I_O(MAX)
=
V_SENSE(MAX)
•
₁₊
1– D_MAX
χ
2^
• R_DS(ON)
•
ρ_T
driving MOSFETs with relatively high package inductance
(DPAK and bigger) or inadequate layout. A small Schottky
diode between NGATE pin and ground can prevent nega-
tive voltage spikes. Two small Schottky diodes can inhibit
positive and negative voltage spikes (Figure 5).
300
IPRG = HIGH
250
200
IPRG = FLOAT
150
100
IPRG = LOW
50
0
1
20
40
60
80
100
DUTY CYCLE (%)
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Figure 3. Maximum SENSE Threshold Voltage vs Duty Cycle
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
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Figure 4. Normalized R_DS(ON) vs Temperature
It is worth noting that the 1 – D_MAX relationship between
I_O(MAX) and R_DS(ON) can cause boost converters with a
wide input range to experience a dramatic range of maxi-
mum input and output current. This should be taken into
consideration in applications where it is important to limit
the maximum current drawn from the input supply.
Voltage on the NGATE pin should be within –0.3V to
(V_IN + 0.3V) limits. Voltage stress below –0.3V and above
V_IN + 0.3V can damage internal MOSFET driver, see Func-
tional Diagram. This is especially important in case of
V_IN
SW
LTC3872-1
NGATE
GND
V_IN
SW
LTC3872-1
NGATE
GND
Figure 5
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