LM34919_14 Datasheet, PDF (8/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
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Control Circuit Overview
The LM34919 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer,
with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below the
reference the buck switch is turned on for a time period determined by the input voltage and a programming
resistor (R_ON). Following the on-time the switch remains off until the FB voltage falls below the reference but not
less than the minimum off-time. The buck switch then turns on for another on-time period. Typically, during start-
up, or when the load current increases suddenly, the off-times are at the minimum. Once regulation is
established, the off-times are longer.
When in regulation, the LM34919 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:
V_OUT x (V_IN ± 1.5V)
F_S = _{1.13 x 10-10 x (RON + 1.4 k:) x VIN}
(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 approximate discontinuous operating frequency can be calculated as
follows:
V_OUT2 x L1 x 1.57 x 10²⁰
F_S =
R_L x (R_ON)²
(3)
where R_L = the load resistance.
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
V_OUT = 2.5 x (R1 + R2) / R2
(4)
Output voltage regulation is based on ripple voltage at the feedback input, normally obtained from the output
voltage ripple through the feedback resistors. The LM34919 requires a minimum of 25 mV of ripple voltage at the
FB pin. In cases where the 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 LM34919. The input pin (VIN) can be connected directly to line voltage up
to 40V, with transient capability to 44V. The V_CC output regulates at 7.0V, and is current limited at 9.5 mA. Upon
power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC
pin reaches the under-voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is
released to allow the Softstart capacitor (C6) to charge up.
The minimum input voltage is determined by the regulator's dropout voltage, the V_CC UVLO falling threshold
(≊5.55V), and the frequency. 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 voltage can be diode connected to the V_CC pin.
Setting the auxiliary voltage to between 7V and 14V 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 52V. Internally, a
diode connects VCC to VIN (see Figure 4).
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