LTC3867_15 Datasheet, PDF (13/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
OPERATION
load across the load capacitors directly greatly benefits
regulation in high current, low voltage applications, where
board interconnection losses can be a significant portion of
the total error budget. Connect DIFF⁺ to the center tap of
the feedback divider across the output load, and DIFF^– to
the load ground. See Figure 1
The LTC3867 differential amplifier has a typical output slew
rate of 2V/µs. The amplifier is configured for unity gain,
meaning that the difference between DIFF⁺ and DIFF^– is
translated to DIFFOUT, relative to SGND.
Care should be taken to route the DIFF⁺ and DIFF^– PCB
traces parallel to each other all the way to the remote sens-
ing points on the board. In addition, avoid routing these
sensitive traces near any high speed switching nodes in
the circuit. Ideally, the DIFF⁺ and DIFF^– traces should be
shielded by a low impedance ground plane to maintain
signal integrity.
Power Good (PGOOD Pin)
The PGOOD pin is connected to the open drain of an
internal N-channel MOSFET. The MOSFET turns on and
pulls the PGOOD pin low when the V_FB pin voltage is not
within ±7.5% of the 0.6V reference voltage. The PGOOD
pin is also pulled low when the RUN pin is below 1.22V or
when the LTC3867 is in the soft-start or tracking up phase.
When the V_FB pin voltage is within the ±7.5% regulation
window, the MOSFET is turned off and the pin is allowed
to be pulled up by an external resistor to a source of up to
6V. The PGOOD pin will flag power good immediately when
the V_FB pin is within the regulation window. However, there
is an internal 45µs power-bad mask when the V_FB goes
out of the window. There is a second set of thresholds set
at 17% and –25% that bypass this delay.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious con-
ditions that may overvoltage the output. In such cases,
the top MOSFET is turned off and the bottom MOSFET is
turned on until the overvoltage condition is cleared.
Undervoltage Lockout
The LTC3867 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV_CC voltage
to ensure that an adequate gate-drive voltage is present. It
locks out the switching action when INTV_CC is below 3.2V.
To prevent oscillation when there is a disturbance on the
INTV_CC, the UVLO comparator has 600mV of precision
hysteresis.
Another way to detect an undervoltage condition is to
monitor the V_IN supply. Because the RUN pin has a pre-
cision turn-on reference of 1.22V, one can use a resistor
divider to V_IN to turn on the IC when V_IN is high enough.
An extra 4µA of current flows out of the RUN pin once
the RUN pin voltage passes 1.22V. The RUN comparator
itself has about 80mV of hysteresis. One can program
additional hysteresis for the RUN comparator by adjust-
ing the values of the resistive divider. For accurate V_IN
undervoltage detection, V_IN needs to be higher than 4.5V.
V_OUT
10Ω
C_OUT2
C_OUT1
R_D1
C_F1
2
DIFF⁺
+
LTC3867
10Ω
R_D2
DIFFAMP
3 ^DIFF– –
DIFFOUT
0Ω
4
V_FB
3867 F01
Figure 1. Differential Amplifier Connection
3867f
13