How to find a faulty item

[Jane]

I need to repair a device that uses a dual synchronous
step-down switching regulator controller LTC3850.
Datasheet is here

Let's suppose it uses the typical application design according to the attached picture.
How would you start to find out the faulty part if there is no
Vout1, nor Vout2? The input voltage Vin is correct.
Thank you for you ideas.

[stj]

Re: How to find a faulty item

check for shorted fets, capacitors etc, then try another chip.

[Jane]

Re: How to find a faulty item

Thanks for the reply. How would you suggest checking shorted FETs?
Desolder them and use a digital multimeter or use a scope in a circuit vith voltage?

[stj]

Re: How to find a faulty item

multimeter should be able to check while they are still on the board.

[Uniballer]

Re: How to find a faulty item

The fact that neither half is working is suspicious. It is less likely that all of the FETs are shorted and more likely that something in common, like the 22uF cap, or the chip itself, is bad.

I tend to start with the scope on a circuit like this. Pick a side. Is there anything at INTVcc (should be about 5V)? Are the TGx and BGx signals oscillating? If so, do you see anything at SWx (even once or a few times after powerup)? Are SENSEx+ and - doing anything (even once, or a few times after powerup)?

[budm]

Re: How to find a faulty item

if you look at page 9 you will see all the notes for the functions of each pin:
PIN FUNCTIONS
RUN1, RUN2 (Pins 1, 13/Pins 26, 9/Pins 27, 10): Run
Control Inputs. A voltage above 1.2V on either pin turns
on the IC. However, forcing either of these pins below 1.2V
causes the IC to shut down that particular channel. There
are 0.5µA pull-up currents for these pins. Once the RUN
pin rises above 1.2V, an additional 4.5µA pull-up current
is added to the pin.
SENSE1+, SENSE2+ (Pins 2, 12/Pins 27, 8/Pins 28, 9):
Current Sense Comparator Inputs. The (+) inputs to the
current comparators are normally connected to DCR sensing
networks or current sensing resistors.
SENSE1–, SENSE2– (Pins 3, 11/Pins 28, 7/Pins 1, 8):
Current Sense Comparator Inputs. The (–) inputs to the
current comparators are connected to the outputs.
TK/SS1, TK/SS2 (Pins 4, 10/Pins 1, 6/Pins 2, 7): Output
Voltage Tracking and Soft-Start Inputs. When one channel
is configured to be master of the two channels, a capacitor
to ground at this pin sets the ramp rate for the master
channel’s output voltage. When the channel is configured
to be the slave of two channels, the VFB voltage of the
master channel is reproduced by a resistor divider and
applied to this pin. Internal soft-start currents of 1.3µA
charge the soft-start capacitors.
ITH1, ITH2 (Pins 5, 9/Pins 2, 5/Pins 3, 6): Current Control
Thresholds and Error Amplifier Compensation Points.
Each associated channels’ current comparator tripping
threshold increases with its ITH control voltage.
VFB1, VFB2 (Pins 6, 8/Pins 3, 4/Pins 4, 5): Error Amplifier
Feedback Inputs. These pins receive the remotely sensed
feedback voltages for each channel from external resistive
dividers across the outputs.
SGND (Pin 7/Pin 29/Pin 29): Signal Ground. All smallsignal
components and compensation components should
connect to this ground, which in turn connects to PGND
at one point. Pin 29 is the Exposed Pad, only available on
the UF package.
EXTVCC (Pin 14, LTC3850-1 Only/Pin 11/Pin 12): External
Power Input to an Internal Switch Connected to INTVCC.
This switch closes and supplies the IC power, bypassing
the internal low dropout regulator, whenever EXTVCC is
higher than 4.7V. Do not exceed 6V on this pin and ensure
VIN > VEXTVCC at all times. On the GN package, EXTVCC
is the optional bonding in place of ILIM for LTC3850-1. In
the LTC3850-1, ILIM will default to 50mV.
ILIM (Pin 14, LTC3850 Only/Pin 10/Pin 11): Current
Comparator Sense Voltage Range Inputs. Tying this pin to
SGND, FLOAT or INTVCC sets the maximum current sense
threshold to three different levels for each comparator.
PGOOD (Pin 15/Pin 12/Pin 13): Power Good Indicator
Output. Open-drain logic out that is pulled to ground when
either channel output exceeds the ±7.5% regulation window,
after the internal 17µs power bad mask timer expires.
PGND (Pin 19/Pin 16/Pin 17): Power Ground Pin. Connect
this pin closely to the sources of the bottom N-channel
MOSFETs, the (–) terminal of CVCC and the (–) terminal
of CIN.
INTVCC (Pin 21/Pin 18/Pin 19): Internal 5V Regulator Output.
The control circuits are powered from this voltage.
Decouple this pin to PGND with a 4.7uF low ESR tantalum
or ceramic capacitor.
VIN (Pin 22/Pin 19/Pin 20): Main Input Supply. Decouple
this pin to PGND with a capacitor (0.1uF to 1uF). For applications
where the main input power is 5V, tie the VIN
and INTVCC pins together.
BG1, BG2 (Pins 23, 20/Pins 20, 17/Pins 21, 18): Bottom
Gate Driver Outputs. These pins drive the gates of the
bottom N-Channel MOSFETs and swings between PGND
and INTVCC.
BOOST1, BOOST2 (Pins 24, 18/Pins 21, 15/Pins 22, 16):
Boosted Floating Driver Supplies. The (+) terminal of the
boost-strap capacitors connect to these pins. These pins
swing from a diode voltage drop below INTVCC up to VIN
+ INTVCC.
TG1, TG2 (Pins 25, 17/Pins 22, 14/Pins 23, 15): Top Gate
Driver Outputs. These are the outputs of floating drivers
with a voltage swing equal to INTVCC superimposed on
the switch nodes voltages.
SW1, SW2 (Pins 26, 16/Pins 23, 13/Pins 24, 14): Switch
Node Connections to Inductors. Voltage swing at these
pins are from a body diode voltage drop below ground
to VIN.

So do you have the actual board so you can find out if all the control signals are present, I.E RUNx?