BUCK — Buck regulator

BUCK is a step-down DC/DC voltage regulator with the following features:

  • High efficiency (low IQ) and low noise operation
  • PWM and Hysteretic modes with automatic switching based on load
  • MODE control pin for forcing PWM mode to minimize output voltage ripple
  • Configurable output voltage between 1.8 V and 3.0 V
When VINT is above VINTBUCKMIN, BUCK is enabled and its output voltage is available at VOUTB.

Hysteretic mode offers efficiency for the full range of supported load currents. PWM mode provides a clean supply operation due to a constant switching frequency, FBUCK. This provides optimal coexistence with RF circuits. BUCK can automatically change between Hysteretic and PWM modes. Modes are controlled by the MODE pin. The state of the MODE pin can be changed at any time.

Parent topic: nPM1100 Product Specification

Output voltage selection (VOUTBSET0, VOUTBSET1)

BUCK output voltage selection pins VOUTBSET0 and VOUTBSET1 should be hardwired to DEC, VSYS, or AVSS. Do not toggle these pins during operation.

Table 1. Output voltage selection
VOUTBSET1 VOUTBSET0 VOUTB voltage
LOW LOW 1.8 V
LOW HIGH 2.1 V
HIGH LOW 2.7 V
HIGH HIGH 3.0 V

For BUCK to supply the desired output voltage, VINT must be VDROPOUT_BUCK greater than the voltage on VOUTB.

When supplied from battery, the following equation gives the VINT, where IBAT is the current being drawn from the battery:

VINT = VBAT – IBAT x RONCHARGER

BUCK mode selection (MODE)

In Automatic mode, BUCK selects Hysteretic mode for low load currents, and PWM mode for high load currents.

This maximizes efficiency over the full range of supported load currents. In PWM mode, BUCK provides a clean supply operation due to constant switching frequency and lower voltage ripple. This allows for optimal coexistence with RF circuits. The MODE pin can be changed at any time.
Table 2. BUCK mode selection
MODE BUCK operation mode
LOW Automatic selection between Hysteretic and PWM modes
HIGH PWM mode

Component selection

Recommended values for the inductor are shown in the following table.

Table 3. Inductor selection
Parameter Value Units
Nominal inductance 2.2 μH
Inductor tolerance ≤ 20 %
DC resistance (DCR) ≤ 400 mΩ
Saturation current (lsat) ≥ 350 mA
Maximum current (lmax) ≥ 350 mA

The following table shows the minimum and maximum effective capacitance at VOUTB.

Table 4. Output capacitor selection
Recommended nominal capacitor Min. Max.
10 µF 6 µF 20 µF

Electrical specification

Table 5. BUCK electrical specification
Symbol Description Min. Typ. Max. Unit
VOUTBACC VOUTB accuracy under static conditions; no change in supply voltage, load current, or Buck operating mode -2 - 8 %
IOUTBSHORT Short circuit current limit - - 400 mA
IPWMTHRES Load current threshold from Hysteretic to PWM mode (MODE = LOW) 90 mA
IHYSTTHRES Load current threshold from PWM to Hysteretic mode (MODE = LOW) 40 mA
VOUTBRIPPLE_PWM VOUTB ripple, MODE = HIGH or load current above IPWMTHRES - - 10 mVpp
VOUTBRIPPLE_HYST VOUTB ripple, MODE = LOW and load current below IPWMTHRES - - 80 mVpp
EFFBUCK Efficiency, VOUTBSET = 11 (VOUTB = 3.0 V), VINT = 3.7 V, IOUTB = 100 mA - 93.5 - %
VDROPOUT_BUCK Dropout voltage, V(VOUTB) - VINT - 0.41 V
FBUCK Switching frequency for PWM mode - 3.6 - MHz
TPWMMODE Hysteretic to PWM mode transition time on MODE pin toggle - - 55 μs
THYSTMODE PWM to Hysteretic mode transition time on MODE pin toggle - - 25 μs
TPWM Hysteretic to PWM mode transition time - - 90 μs
THYST PWM to Hysteretic mode transition time - - 35 μs
TSETTLE Settling time to within 1% after load transient of 0 A to 100 mA - - 20 μs
VINTBUCKMIN Minimum VINT voltage for enabling BUCK - 2.8 - V

Electrical characteristics

The following graphs show BUCK electrical characteristics.

Figure 1. VOUTB=3.0 system efficiency, MODE=AUTO
VOUTB=3.0 system efficiency, MODE=AUTO
Figure 2. VOUTB=3.0 system efficiency, MODE=PWM
VOUTB=3.0 system efficiency, MODE=PWM
Figure 3. VOUTB=3.0: VOUTB vs. temperature (VBAT=4.2)
VOUTB=3.0: VOUTB vs. Temperature (VBAT = 4.2)
Figure 4. VOUTB=2.7 system efficiency, MODE=AUTO
VOUTB=2.7 system efficiency, MODE=AUTO
Figure 5. VOUTB=2.7 system efficiency, MODE=PWM
VOUTB=2.7 system efficiency, MODE=PWM
Figure 6. VOUTB=2.1 system efficiency, MODE=AUTO
VOUTB=2.1 system efficiency, MODE=AUTO
Figure 7. VOUTB=2.1 system efficiency, MODE=PWM
VOUTB=2.1 system efficiency, MODE=PWM
Figure 8. VOUTB=1.8 system efficiency, MODE=AUTO
VOUTB=1.8 system efficiency, MODE=AUTO
Figure 9. VOUTB=1.8 system efficiency, MODE=PWM
VOUTB=1.8 system efficiency, MODE=PWM
Figure 10. VOUTB=1.8 VOUTB vs. temperature (VBAT=4.2)
VOUTB=1.8: VOUTB vs. Temperature (VBAT=4.2)
Figure 11. Startup with no load, soft start, Vout=1.8 V, VBAT=3.8 V
Startup with no load, soft start, Vout=1.8 V, VBAT=3.8 V
Figure 12. BUCK load transition in auto mode (MODE=0), Iout=10 mA → 150 mA → 10 mA (1 µs step), Vout=1.8 V, VBAT=3.8 V
BUCK Load tran in auto mode (MODE=0), Iout=10 mA->150 mA->10 mA (1 us step), Vout=1.8 V, VBAT=3.8 V
Figure 13. BUCK Mode transition, MODE pin 0 → 1, Vout=1.8 V Iout=10 mA
BUCK Mode transition, MODE pin 0>1, Vout=1.8 V Iout=10 mA
Figure 14. BUCK Mode transition, MODE pin 1 → 0, Vout=1.8 V Iout=10 mA
BUCK Mode transition, MODE pin 1->0, Vout=1.8 V Iout=10 mA
Figure 15. BUCK load transition in PWM mode (MODE=1), Iout=10 mA → 150 mA → 10 mA (1 µs step), Vout=1.8 V, VBAT=3.8 V
BUCK Load tran in PWM mode (MODE=1), Iout=10 mA>150 mA>10 mA (1 us step), Vout=1.8 V, VBAT=3.8 V