LM3475_14 Datasheet, PDF (9/21 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3475
www.ti.com
SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005
At very low or very high duty cycle operation, maximum frequency will be limited by the blanking time. The
maximum operating frequency can be determined by the following equations:
F_MAX = D / ton_min
(1)
F_MAX = (1-D) / toff_min
where
• D is the duty cycle, defined as V_OUT/V_IN, and ton_min
• toff_min is the sum of the blanking time, the propagation delay time, and the PFET delay time (see Figure 12) (2)
ENABLE PIN (EN)
The LM3475 provides a shutdown function via the EN pin to disable the device. The device is active when the
EN pin is pulled above 1.5V (typ) and in shutdown mode when EN is below 1.135V (typ). In shutdown mode,
total quiescent current is less than 10µA. The EN pin can be directly connected to V_IN for always-on operation.
Design Information
SETTING OUTPUT VOLTAGE
The output voltage is programmed using a resistor divider between V_OUT and GND as shown in Figure 13. The
feedback resistors can be calculated as follows:
R₁ + R₂
V_OUT =
R₂ ^{x VFB}
where
• Vfb is 0.8V typically
(3)
The feedback resistor ratio, α = (R1+R2) / R2, will also be used below to calculate output ripple and operating
frequency.
PGATE
PMOS_drv
L
V_OUT
+
C_OUT
R1
C_ff
+
-
Hyst Comp
FB
R2
+
Reference ^-
Voltage
V_HYST = 21 mV
V_FB = 0.8V
PGATE
Figure 13. Hysteretic Window
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