LTC3808 Datasheet, PDF (12/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
U
OPERATIO (Refer to Functional Diagram)
maximum value of V_ITH is typically about 1.98V, so the
maximum sense voltage allowed across the external
P-channel MOSFET is 125mV, 85mV or 204mV for the
three respective states of the IPRG pin.
However, once the controller’s duty cycle exceeds 20%,
slope compensation begins and effectively reduces the
peak sense voltage by a scale factor (SF) given by the curve
in Figure 1.
The peak inductor current is determined by the peak sense
voltage and the on-resistance of the external P-channel
MOSFET:
I_PK
=
∆V_SENSE(MAX)
R_DS(ON)
If a sense resistor is used, ∆V_SENSE(MAX) is the peak
current sense voltage (between the SENSE⁺ and SENSE^–
pins) across the sense resistor. The peak inductor is
determined by the peak sense voltage and the resistance
of the sense resistor:
I_PK
=
∆V_SENSE(MAX)
R_SENSE
Power Good (PGOOD) Pin
A window comparator monitors the feedback voltage and
the open-drain PGOOD output pin is pulled low when the
feedback voltage is not within ±10% of the 0.6V reference
voltage. PGOOD is low when the LTC3808 is shut down or
in undervoltage lockout.
110
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3808 F01
Figure 1. Maximum Peak Current vs Duty Cycle
3808f
12