LM34919C-Q1 Datasheet, PDF (13/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
www.ti.com
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
When in regulation, the LM34919C operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode current always flows
through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains
relatively constant with load and line variations. The minimum load current for continuous conduction mode is
one-half the inductor’s ripple current amplitude. The operating frequency is approximately:
F_S

V_OUT
R_ON K 35.5 K10^-12
Hz
(1)
The buck switch duty cycle is approximately equal to:
t_ON
V_OUT
DC =
=
t_ON + t_OFF V_IN
(2)
In discontinuous conduction mode, current through the inductor ramps up from zero to a peak during the on-time,
then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB
falls below the reference. Until then the inductor current remains zero, and the load current is supplied by the
output capacitor. In this mode the operating frequency is lower than in continuous conduction mode, and varies
with load current. Conversion efficiency is maintained at light loads since the switching losses decrease with the
reduction in load and frequency.
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
V_OUT = 2.52 x (R1 + R2) / R2
(3)
Output voltage regulation is based on ripple voltage at the feedback input, normally obtained from the output
voltage ripple through the feedback resistors. The LM34919C requires a minimum of 25 mV of ripple voltage at
the FB pin. In cases where the output capacitor’s ESR is insufficient additional series resistance may be required
(R3).
Start-Up Regulator, V_CC
The start-up regulator is integral to the LM34919C. The input pin (VIN) can be connected directly to line voltage
up to 50 V with transient capability to 65 V. The V_CC output regulates at 7.0 V and is current limited at 27 mA.
Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the
VCC pin reaches the undervoltage lockout rising threshold of 3.75 V, the buck switch is enabled and the soft-
start pin is released to allow the soft-start capacitor (C6) to charge up.
The minimum input voltage is determined by the V_CC UVLO falling threshold (≊3.6 V). When V_CC falls below the
falling threshold the V_CC UVLO activates to shut off the output. If V_CC is externally loaded, the minimum input
voltage increases.
To reduce power dissipation in the start-up regulator, an auxiliary bias voltage can be diode connected to the V_CC
pin (see Figure 7). Setting the auxiliary bias voltage between 7.6 V and 14 V shuts off the internal regulator
reducing internal power dissipation. The sum of the auxiliary voltage and the input voltage (V_CC + V_IN) cannot
exceed 79 V. An internal diode connects VCC to VIN. (See Figure 5).
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