lm26003mhx National Semiconductor Corporation, lm26003mhx Datasheet - Page 8

• Manufacturer Part Number: lm26003mhx
• Description: 3a Switching Regulator With High Efficiency Sleep Mode
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
GENERAL
The LM26003 is a current mode PWM buck regulator. At the
beginning of each clock cycle, the internal high-side switch
turns on, allowing current to ramp-up in the inductor. The in-
ductor current is internally monitored during each switching
cycle. A control signal derived from the inductor current is
compared to the voltage control signal at the COMP pin, de-
rived from the feedback voltage. When the inductor current
reaches its threshold, the high-side switch is turned off and
inductor current ramps-down. While the switch is off, inductor
current is supplied through the catch diode. This cycle repeats
at the next clock cycle. In this way, duty-cycle and output volt-
age are controlled by regulating inductor current. Current
mode control provides superior line and load regulation. Other
benefits include cycle-by-cycle current limiting and a simpli-
fied compensation scheme. Typical PWM waveforms are
shown in Figure 1.
SLEEP MODE
In light load conditions, the LM26003 automatically switches
into sleep mode for improved efficiency. As loading decreas-
es, the voltage at FB increases and the COMP voltage de-
creases. When the COMP voltage reaches the 0.6V (typical)
clamp threshold and the FB voltage rises 1% above nominal,
sleep mode is enabled and switching stops. The regulator re-
mains in sleep mode until the FB voltage falls to the reset
threshold, at which point switching resumes. This 1% FB win-
dow limits the corresponding output ripple requirement to
approximately 1% of nominal output voltage. The sleep cycle
will repeat until load current is increased. Figure 2 shows typ-
ical switching and output voltage waveforms in sleep mode.
FIGURE 1. PWM Waveforms
1A Load, Vin = 12V
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In sleep mode, quiescent current is reduced to less than 40
µA (typical) when not switching. The DC sleep mode thresh-
old can roughly be calculated according to the equation be-
low:
Where Imin = Ilim/16 (4.7A/16 typically) and D = duty-cycle,
defined as (Vout+Vdiode)/Vin.
When load current increases above this limit, the LM26003 is
forced back into PWM operation. The sleep mode threshold
varies with frequency, inductance, and duty-cycle as shown
in Figure 3.
FPWM
Pulling the FPWM pin high disables sleep mode and forces
the LM26003 to always operate in PWM mode. Light load ef-
ficiency is reduced in PWM mode, but switching frequency
remains stable. The FPWM pin can be connected to the VDD
pin to pull it high. In FPWM mode, under light load conditions,
the regulator operates in discontinuous conduction mode
(DCM) . In discontinuous conduction mode, current through
FIGURE 3. Sleep Mode Threshold vs Vin
FIGURE 2. Sleep Mode Waveforms
25 mA Load, Vin = 12V
Vout = 3.3V
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