
LT3956
PIN FUNCTIONS
V REF : Voltage Reference Output Pin (typically 2V). This
pin drives a resistor divider for the CTRL pin, either for
analog dimming or for temperature limit/compensation
of LED load. Can supply up to 100μA.
PWM: A signal low turns off switcher, idles oscillator and
disconnects V C pin from all internal loads. PWMOUT pin
follows PWM pin. PWM has an internal pull-down resistor.
If not used, connect to INTV CC .
VMODE : An open-collector pull-down on VMODE asserts
if the FB input is greater than the FB regulation threshold
minus 50mV (typical). To function, the pin requires an
external pull-up resistor. When the PWM input is low
and the DC/DC converter is idle, the VMODE condition is
latched to the last valid state when the PWM input was
BLOCK DIAGRAM
high. When PWM input goes high again, the VMODE pin
will be updated. This pin may be used to report an open
LED fault. Use a pull-up current less than 1mA.
SS: Soft-Start Pin. This pin modulates oscillator fre-
quency and compensation pin voltage (V C ) clamp. The
soft-start interval is set with an external capacitor. The
pin has a 10μA (typical) pull-up current source to an
internal 2.5V rail. The soft-start pin is reset to GND by
an undervoltage condition (detected by EN/UVLO pin)
or thermal limit.
RT: Switching Frequency Adjustment Pin. Set the fre-
quency using a resistor to GND (for resistor values, see
the Typical Performance curve or Table 1). Do not leave
the RT pin open.
EN/UVLO
A6
FB
VC
PWMOUT
PWM
V IN
1.22V
2.1μA
SHDN
1.31V
OVFB
COMPARATOR
1.25V
LDO
A8
7.15V
INTV CC
+
–
1.25V
A5
g m
10μA AT
FB = 1.25V
SW
+
+
–
–
+
–
+
–
–
+
–
ISN
ISP
SHORT-CIRCUIT
DETECT
A10
350mV
CTRL
CTRL BUFFER
1.1V + A3
5k
SCILMB
g m
EAMP
A1
Q2
SCILMB
10μA
10μA AT
A1 + = A1 –
VC
SSCLAMP
+
A2
PWM
COMPARATOR
RAMP
GENERATOR
R
S
Q
DRIVER
I SENSE
A4
PGND
GND
+
V REF
2V
A7
20k
140μA
170k
FAULT
LOGIC
TSD
165°C
10μA
1.25V
+
–
100kHz TO 1MHz
OSCILLATOR
FREQ
PROG
1.2V
FB
VMODE
1mA (MAX)
SS
RT
3956f